Summary

The mutation response wrapper is normally named {trainName}Response. When a user's output CLR class happens to be named {trainName}Response (e.g. IAddressValidationTrain returning AddressValidationResponse), HotChocolate gets two type registrations claiming the same GraphQL name and schema build fails with "type registered twice."

The fix detects the collision against the set of output ObjectType names and falls back to {trainName}MutationResponse. Trains whose output types follow a different naming convention are unaffected.

The collision needs three things to line up: the train's derived name ends right before where Response would go, AND the user's output CLR class is literally named {TrainName}Response. Easy footgun because FooResponse is a common convention for output DTOs and IFooTrain → Foo is the standard derivation.

Test plan

• New regression tests in TrainTypeModuleTests.cs under Response Type Name Collision region:
  • Collision case → wrapper renamed; both types present in module output
  • Collision case → real schema build via IRequestExecutorResolver succeeds (the actual failure mode)
  • Non-collision case → default {trainName}Response name preserved
  • Cross-train collision (TrainA's wrapper name matches TrainB's output class name) → still renamed
  • Query train with same shape → no rename (queries don't get wrappers)
• Full Trax.Api.Tests suite passes (1348 tests)
• csharpier-clean
• Companion docs PR in Trax.Docs documenting the fallback