Rename NeedsDefaultConstraint to HasAnnotatedNullableUsage

The model property now describes what was detected (nullable annotation
on an unconstrained type parameter) rather than what action to take.
The decision to emit `where T : default` remains in the builder layer,
gated by the forExplicitImplementation flag.

TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs

@@ -1122,7 +1122,7 @@ private static string FormatConstraintClauses(EquatableArray<MockTypeParameterMo
             {
                 clauses.Add($"where {tp.Name} : {tp.Constraints}");
             }
-            else if (forExplicitImplementation && tp.NeedsDefaultConstraint)
+            else if (forExplicitImplementation && tp.HasAnnotatedNullableUsage)
             {
                 clauses.Add($"where {tp.Name} : default");
             }

TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs

@@ -507,7 +507,7 @@ private static MockMemberModel CreateMethodModel(IMethodSymbol method, ref int m
                 {
                     Name = tp.Name,
                     Constraints = tp.GetGenericConstraints(),
-                    NeedsDefaultConstraint = tp.IsUnconstrainedWithNullableUsage(method)
+                    HasAnnotatedNullableUsage = tp.IsUnconstrainedWithNullableUsage(method)
                 }).ToImmutableArray()
             ),
             ExplicitInterfaceName = explicitInterfaceName,

TUnit.Mocks.SourceGenerator/Models/MockTypeParameterModel.cs

@@ -6,12 +6,12 @@ internal sealed record MockTypeParameterModel : IEquatable<MockTypeParameterMode
 {
     public string Name { get; init; } = "";
     public string Constraints { get; init; } = "";
-    public bool NeedsDefaultConstraint { get; init; }
+    public bool HasAnnotatedNullableUsage { get; init; }
 
     public bool Equals(MockTypeParameterModel? other)
     {
         if (other is null) return false;
-        return Name == other.Name && Constraints == other.Constraints && NeedsDefaultConstraint == other.NeedsDefaultConstraint;
+        return Name == other.Name && Constraints == other.Constraints && HasAnnotatedNullableUsage == other.HasAnnotatedNullableUsage;
     }
 
     public override int GetHashCode()
@@ -21,7 +21,7 @@ public override int GetHashCode()
             int hash = 17;
             hash = hash * 31 + Name.GetHashCode();
             hash = hash * 31 + Constraints.GetHashCode();
-            hash = hash * 31 + NeedsDefaultConstraint.GetHashCode();
+            hash = hash * 31 + HasAnnotatedNullableUsage.GetHashCode();
             return hash;
         }
     }