[Kotlin][jvm-spring-restclient] make nullableReturnType type bound conditional in ApiClient

[takkiraz]

Fixes #23654

PR #23613 unconditionally relaxed the type bound from T : Any to T : Any? in ApiClient.kt.mustache for jvm-spring-restclient. This breaks compilation with Spring Boot 4 / Spring Framework 7, where stricter nullability handling rejects nullable bounds on RestClient.ResponseSpec.toEntity().

Make the relaxation conditional on the nullableReturnType option:

Config	T bound	Notes
Default (nullableReturnType=false)	T : Any	Identical to pre-#23613
nullableReturnType=true	T : Any?	Preserves the #23613 NPE fix

Also restores I bound to Any everywhere (request bodies are never nullable) and removes the unnecessary as Any cast in nullableBody.

Verification:

• Reproduced the failure on master with Spring Boot 4.0.6 + Kotlin 2.3.21
• Confirmed this fix resolves the compilation error
• Original NPE fix from [kotlin][jvm-spring-restclient] honour nullableReturnType in body access #23613 remains intact (regression test in nullable-return sample passes)
• ./bin/generate-samples.sh bin/configs/kotlin-*.yaml — 112 generators, zero drift
• ./bin/utils/export_docs_generators.sh — no doc changes

Note: nullableReturnType=true combined with Spring Boot 4 still has a separate compatibility issue with Spring Framework 7's toEntity() bounds. That requires a different architectural approach and is being considered as a follow-up; out of scope for this regression fix.

cc @wing328 @stefankoppier @karismann @Zomzog @andrewemery @4brunu @yutaka0m @e5l @dennisameling

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work.
• Run the following to build the project and update samples:
  • ./bin/generate-samples.sh bin/configs/kotlin-*.yaml — 112 generators, zero drift
  • ./bin/utils/export_docs_generators.sh — no doc changes
  • All three restclient samples build successfully; nullable-return regression tests pass
• File the PR against the correct branch: master
• PR references issue using GitHub's linking syntax (fixes [BUG] Regression of 7.21.0 -> 7.22.0 Generated Kotlin code does not compile #23654)
• @mention the technical committee members

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Summary by cubic

Conditionally relax the response type bound in jvm-spring-restclient ApiClient.kt.mustache based on nullableReturnType. Restores Spring Boot 4 / Spring Framework 7 compatibility while keeping the nullable-return NPE fix.

• Bug Fixes
  • Default (nullableReturnType=false): T : Any; when true: T : Any?, avoiding RestClient.ResponseSpec.toEntity() bound errors.
  • Restore request body bound to non-null (I : Any) and remove the unnecessary as Any cast.
  • Verified: samples build with Spring Boot 4 + Kotlin 2.3.21; nullable-return regression passes; no doc or sample drift.

^{Written for commit c79ab9b. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files