Fix CS0121 ambiguity in generated Program.g.cs for no-input Task handlers

[GarrettBeatty]

Summary

Fixes #1907

When a Lambda handler has no input parameters and returns Task (e.g., public async Task FunctionHandler()), the Annotations source generator produced ambiguous code in Program.g.cs:

Func<Stream, Task> handler = new ...().FunctionHandler;
await LambdaBootstrapBuilder.Create(handler, new Serializer()).Build().RunAsync();

Func<Stream, Task> matches two overloads, causing CS0121:

• Create<TInput>(Func<TInput, Task>, ILambdaSerializer) — TInput = Stream
• Create<TOutput>(Func<Stream, TOutput>, ILambdaSerializer) — TOutput = Task

Fix

In ExecutableAssembly.tt/.cs, detect this specific case (no typed input + Task return) and use the non-generic, non-serializer overload Create(Func<Stream, Task>), which is unambiguous and correct since no deserialization is needed when there is no typed input:

Func<Stream, System.Threading.Tasks.Task> handler = new ...().FunctionHandler;
await LambdaBootstrapBuilder.Create(handler).Build().RunAsync();

Testing

Added VerifyExecutableAssemblyWithParameterlessTaskReturnConstructor snapshot test covering the exact reproduction case from the issue.

[Copilot]

Pull request overview

This PR fixes the Annotations source generator so executable Lambda projects with parameterless Task handlers generate an unambiguous LambdaBootstrapBuilder.Create call, resolving CS0121 for the issue reproduction.

Changes:

• Adds a template branch for parameterless Task handlers to use Create(Func<Stream, Task>) without a serializer.
• Updates the generated T4 backing class accordingly.
• Adds a regression source file, snapshots, and a verifier test for the parameterless async handler case.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Templates/ExecutableAssembly.tt	Adds the unambiguous bootstrap generation path.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Templates/ExecutableAssembly.cs	Updates generated T4 implementation for the template change.
Libraries/test/TestExecutableServerlessApp/ParameterlessTaskMethods.cs	Adds a parameterless Task handler test input.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SourceGeneratorTests.cs	Adds the regression verification test.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ProgramParameterlessTask.g.cs	Captures expected generated executable program output.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ParameterlessTaskMethods_NoParameterTask_Generated.g.cs	Captures expected generated handler wrapper output.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ServerlessTemplates/parameterlesstask.template	Captures expected serverless template output.
.autover/changes/23b26fb3-e30b-47a7-8a8e-3d8504420311.json	Adds patch changelog metadata.

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