Add cross-cutting types and align design doc with reference SDKs

Lays down shared types/constants for the upcoming durable-execution
context operations (Callbacks, Invoke, Parallel, Map, WaitForCondition)
and updates the design doc to match decisions reached after comparing
against the Python, JS, and Java reference SDKs.

SDK changes:
- OperationSubTypes constants class (Step, Wait, Callback, WaitForCallback,
  Invoke, WaitForCondition, Parallel, ParallelBranch, Map, MapIteration).
  Replaces hard-coded SubType literals in StepOperation and WaitOperation.
- OperationStatuses.TimedOut for callback/invoke timeout handling.

Design-doc alignment:
- Drop Serializer field from CallbackConfig, InvokeConfig,
  ChildContextConfig. Custom serializers flow through AOT-safe
  ICheckpointSerializer<T> overloads (matches the existing StepConfig
  pattern documented at line 1247).
- InvokeConfig gains TenantId (matches Python/JS/Java); drops
  PayloadSerializer / ResultSerializer.
- BatchItemStatus.Cancelled -> Started. The SDK does not synchronously
  cancel branches; the wire state of items still in flight when the
  batch resolves (e.g., FirstSuccessful short-circuit) is STARTED.
  Matches Python and JS.
- IBatchResult<T> expanded to the full JS/Python surface: adds Started,
  GetErrors(), HasFailure, SuccessCount, FailureCount, StartedCount,
  TotalCount.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: GarrettBeatty

Docs/durable-execution-design.md

@@ -1279,10 +1279,9 @@ public class CallbackConfig
     /// </summary>
     public TimeSpan HeartbeatTimeout { get; set; } = TimeSpan.Zero;
 
-    /// <summary>
-    /// Custom serializer for callback result.
-    /// </summary>
-    public ICheckpointSerializer? Serializer { get; set; }
+    // Note: there is no Serializer property here. Custom serializers are
+    // supplied via the AOT-safe CreateCallbackAsync(..., ICheckpointSerializer<T>, ...)
+    // overload, matching the pattern established by StepAsync.
 }
 
 /// <summary>
@@ -1307,14 +1306,14 @@ public class InvokeConfig
     public TimeSpan Timeout { get; set; } = TimeSpan.Zero;
 
     /// <summary>
-    /// Custom serializer for the payload.
+    /// Optional tenant identifier propagated to the chained invocation.
+    /// Matches the tenantId field on Python/JS/Java InvokeConfig.
     /// </summary>
-    public ICheckpointSerializer? PayloadSerializer { get; set; }
+    public string? TenantId { get; set; }
 
-    /// <summary>
-    /// Custom serializer for the result.
-    /// </summary>
-    public ICheckpointSerializer? ResultSerializer { get; set; }
+    // Note: payload and result serializers are supplied via the AOT-safe
+    // InvokeAsync(..., ICheckpointSerializer<TPayload>, ICheckpointSerializer<TResult>, ...)
+    // overload, matching the pattern established by StepAsync.
 }
 
 /// <summary>
@@ -1429,10 +1428,9 @@ public class CompletionConfig
 /// </summary>
 public class ChildContextConfig
 {
-    /// <summary>
-    /// Custom serializer for the child context's return value.
-    /// </summary>
-    public ICheckpointSerializer? Serializer { get; set; }
+    // Note: there is no Serializer property here. Custom serializers are
+    // supplied via the AOT-safe RunInChildContextAsync(..., ICheckpointSerializer<T>, ...)
+    // overload, matching the pattern established by StepAsync.
 
     /// <summary>
     /// Operation sub-type label for observability (e.g., in test runner output).
@@ -1473,34 +1471,54 @@ public class WaitForConditionConfig<TState>
 public interface IBatchResult<T>
 {
     /// <summary>
-    /// All items (succeeded and failed).
+    /// All items, in original index order.
     /// </summary>
     IReadOnlyList<IBatchItem<T>> All { get; }
 
     /// <summary>
-    /// Only successful items.
+    /// Items whose Status is Succeeded.
     /// </summary>
     IReadOnlyList<IBatchItem<T>> Succeeded { get; }
 
     /// <summary>
-    /// Only failed items.
+    /// Items whose Status is Failed.
     /// </summary>
     IReadOnlyList<IBatchItem<T>> Failed { get; }
 
     /// <summary>
-    /// Get all successful results. Throws if any failed.
+    /// Items still in flight when the batch resolved (CompletionConfig short-circuit).
+    /// </summary>
+    IReadOnlyList<IBatchItem<T>> Started { get; }
+
+    /// <summary>
+    /// Get all successful results in original index order. Throws if any failed.
     /// </summary>
     IReadOnlyList<T> GetResults();
 
     /// <summary>
-    /// Throw an exception if any item failed.
+    /// Get all errors from failed items.
+    /// </summary>
+    IReadOnlyList<DurableExecutionException> GetErrors();
+
+    /// <summary>
+    /// Throw a single aggregated exception if any item failed.
     /// </summary>
     void ThrowIfError();
 
     /// <summary>
-    /// Why the operation completed.
+    /// True if any item is in the Failed state.
+    /// </summary>
+    bool HasFailure { get; }
+
+    /// <summary>
+    /// Why the batch resolved.
     /// </summary>
     CompletionReason CompletionReason { get; }
+
+    int SuccessCount { get; }
+    int FailureCount { get; }
+    int StartedCount { get; }
+    int TotalCount { get; }
 }
 
 public interface IBatchItem<T>
@@ -1511,7 +1529,29 @@ public interface IBatchItem<T>
     DurableExecutionException? Error { get; }
 }
 
-public enum BatchItemStatus { Succeeded, Failed, Cancelled }
+/// <summary>
+/// Status of an individual item in a batch result.
+/// Mirrors the wire-state observed at the time the batch resolved — items still
+/// running when a CompletionConfig short-circuits remain in <see cref="Started"/>.
+/// </summary>
+public enum BatchItemStatus
+{
+    /// <summary>
+    /// The branch ran to completion and produced a result.
+    /// </summary>
+    Succeeded,
+
+    /// <summary>
+    /// The branch ran to completion and threw.
+    /// </summary>
+    Failed,
+
+    /// <summary>
+    /// The branch was still in flight when the batch's CompletionConfig
+    /// resolved (e.g., FirstSuccessful returned before this branch finished).
+    /// </summary>
+    Started
+}
 public enum CompletionReason { AllCompleted, MinSuccessfulReached, FailureToleranceExceeded }
 
 /// <summary>

Libraries/src/Amazon.Lambda.DurableExecution/Internal/Operation.cs

@@ -137,4 +137,25 @@ internal static class OperationStatuses
     public const string Cancelled = "CANCELLED";
     public const string Ready = "READY";
     public const string Stopped = "STOPPED";
+    public const string TimedOut = "TIMED_OUT";
+}
+
+/// <summary>
+/// Wire-format <see cref="Operation.SubType"/> string constants. Subtypes are
+/// observability labels mapped from the user-facing context method that
+/// produced the operation. The service does not interpret them; downstream
+/// consumers (test runner, traces, console) display them as-is.
+/// </summary>
+internal static class OperationSubTypes
+{
+    public const string Step = "Step";
+    public const string Wait = "Wait";
+    public const string Callback = "Callback";
+    public const string WaitForCallback = "WaitForCallback";
+    public const string Invoke = "Invoke";
+    public const string WaitForCondition = "WaitForCondition";
+    public const string Parallel = "Parallel";
+    public const string ParallelBranch = "ParallelBranch";
+    public const string Map = "Map";
+    public const string MapIteration = "MapIteration";
 }

Libraries/src/Amazon.Lambda.DurableExecution/Internal/StepOperation.cs

@@ -171,7 +171,7 @@ private async Task<T> ExecuteFunc(int attemptNumber, CancellationToken cancellat
                 Id = OperationId,
                 Type = OperationTypes.Step,
                 Action = "START",
-                SubType = "Step",
+                SubType = OperationSubTypes.Step,
                 Name = Name
             };
 
@@ -196,7 +196,7 @@ await EnqueueAsync(new SdkOperationUpdate
                 Id = OperationId,
                 Type = OperationTypes.Step,
                 Action = "SUCCEED",
-                SubType = "Step",
+                SubType = OperationSubTypes.Step,
                 Name = Name,
                 Payload = SerializeResult(result)
             }, cancellationToken);
@@ -233,7 +233,7 @@ await EnqueueAsync(new SdkOperationUpdate
                     Id = OperationId,
                     Type = OperationTypes.Step,
                     Action = "RETRY",
-                    SubType = "Step",
+                    SubType = OperationSubTypes.Step,
                     Name = Name,
                     Error = ToSdkError(ex),
                     StepOptions = new SdkStepOptions { NextAttemptDelaySeconds = delaySeconds }
@@ -248,7 +248,7 @@ await EnqueueAsync(new SdkOperationUpdate
             Id = OperationId,
             Type = OperationTypes.Step,
             Action = "FAIL",
-            SubType = "Step",
+            SubType = OperationSubTypes.Step,
             Name = Name,
             Error = ToSdkError(ex)
         }, cancellationToken);

Libraries/src/Amazon.Lambda.DurableExecution/Internal/WaitOperation.cs

@@ -48,7 +48,7 @@ await EnqueueAsync(new SdkOperationUpdate
             Id = OperationId,
             Type = OperationTypes.Wait,
             Action = "START",
-            SubType = "Wait",
+            SubType = OperationSubTypes.Wait,
             Name = Name,
             WaitOptions = new SdkWaitOptions { WaitSeconds = _waitSeconds }
         }, cancellationToken);