[swift6][urlsession] Deliver successful responses on apiResponseQueue

[spigo]

Fixes #25204.

The urlsession request builder delivered failures on apiResponseQueue (the main queue by default), but called the completion of successful responses directly on URLSession's delegate queue. This change sends the result of processRequestResponse through apiResponseQueue as well, so every result arrives where callers expect it. Decoding still happens on URLSession's queue; only the delivery moves. The retry and failure paths already dispatched to apiResponseQueue, so nothing is dispatched twice.

This also fixes the next sample-test failure on Bitrise after #25203: the combineLibrary test class is @MainActor, so its Combine sink closures crashed under Swift 6's runtime isolation check when a successful response arrived off the main thread.

Verified with the generated combineLibrary client against https://petstore.swagger.io: before, a successful addPet and getPetById completed off the main thread and a 404 on it, and a @MainActor sink logged data race detected: @MainActor function ... was not called on the main thread. After, all three complete on the main thread and the sink runs without the warning. The default, urlsessionLibrary and combineLibrary samples build.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/swift6*.yaml || exit
  

  Commit all changed files.
• @mention the technical committee: @jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

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Summary by cubic

Fixes #25204 by delivering successful URLSession responses on apiResponseQueue, matching the failure path. Previously failures were dispatched to apiResponseQueue while successful responses completed directly on URLSession's delegate queue, which made @MainActor Combine sinks run off the main thread and trip Swift 6's runtime isolation check. Decoding still happens on URLSession's queue; only delivery moves. The retry and failure paths already dispatched to apiResponseQueue, so nothing is dispatched twice. This also fixes the combineLibrary sample-test failure on Bitrise.

^{Written for commit 28757a5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 12 files

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