[BUG][swift6][urlsession] Successful responses ignore apiResponseQueue and are delivered on a background thread #25204

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator? (not spec-specific)
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

The urlsession library delivers failures on apiResponseQueue (the main queue by default), but successful responses are delivered directly on URLSession's delegate queue: the data task's completion calls processRequestResponse(...), which calls completion(.success(...)) without going through apiResponseQueue. So, with the default configuration, an app gets errors on the main thread and results on a background thread.

Code that relies on main-queue delivery then runs off the main thread. Under Swift 6 a @MainActor closure called that way trips the runtime isolation check. This is what currently crashes the combineLibrary sample tests on Bitrise: the @MainActor test class's sink closures receive the value off main, the test process traps and is restarted for every test, and the run ends with "Executed 0 tests" / TEST EXECUTE FAILED.

openapi-generator version

master (57164fa). Not a regression.

OpenAPI declaration file content or url

Not spec-specific; reproducible with the swift6 samples' petstore spec.

Generation Details

Any swift6 config with library: urlsession (the default), e.g. bin/configs/swift6-combineLibrary.yaml or bin/configs/swift6-default.yaml.

Steps to reproduce

With the generated combineLibrary client and the default configuration:

• PetAPI.addPetWithRequestBuilder(body:).execute { _ in print(Thread.isMainThread) } prints false on success
• PetAPI.getPetByIdWithRequestBuilder(petId: 1).execute { ... } for a missing pet (404) prints true
• A @MainActor PetAPI.getPetById(petId:).sink(...) logs data race detected: @MainActor function ... was not called on the main thread (a crash in an iOS app under Swift 6)

Expected: every result is delivered on apiResponseQueue.

Related issues/PRs

#25203 fixes the earlier sample-test failure (http redirect); with it, combineLibrary is the first sample whose tests fail.

Suggest a fix

Deliver the result of processRequestResponse on apiResponseQueue, as the failure paths already do; decoding can stay on URLSession's queue. PR follows.