[Kotlin][Spring] use WebFlux Part types for reactive multipart files

[wingwogus]

Fixes #21548.

Summary

• Generate WebFlux multipart file parameters as org.springframework.http.codec.multipart.Part for reactive Kotlin Spring Boot servers.
• Generate reactive file arrays as reactor.core.publisher.Flux<Part>.
• Preserve the existing MultipartFile / Array<MultipartFile> output for blocking Spring Boot and reactive declarative HTTP interfaces.
• Preserve nullability and all non-file multipart parameter rendering.

Implementation notes

A private derived template flag is enabled only for reactive=true with the spring-boot library. The existing optionalDataType.mustache file branch uses that flag, so this remains a narrow type-system correction rather than a new public generator option or a broad template split.

The two checked-in reactive Kotlin Spring samples were regenerated. Their intended deltas are limited to the multipart parameter type in the controller, service, service implementation, and generated API test.

@dr4ke616 @e5l, this touches the Kotlin Spring Boot reactive server output.

Verification

• Full KotlinSpringServerCodegenTest: 236 tests, 0 failures/errors/skips.
• Regenerated kotlin-spring-boot-reactive and kotlin-spring-boot-reactive-without-flow samples.
• Compiled both sample main source sets on local JDK 21 with Gradle 8.5.
• Compared sample test compilation with clean base copies: the multipart mismatch is removed and no new error is introduced; unrelated pre-existing stale errors remain in both base and modified copies.
• checkstyle:check passed for modules/openapi-generator.
• git diff --check passed.

The official JDK 11 / Gradle 7.6.4 sample matrix was not available locally; CI is expected to cover it.

PR checklist

• Read the contribution guidelines.
• Run the full repository build/sample/doc regeneration commands from the PR template. The two affected samples, full generator test class, sample compilation, and checkstyle were run as documented above.
• Mention the relevant technical committee members.

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Summary by cubic

Use WebFlux Part for reactive multipart parameters in Kotlin Spring Boot servers, with arrays as Flux<Part>, to match the reactive request pipeline. Blocking Spring Boot and declarative HTTP interfaces keep MultipartFile. Fixes #21548.

• Bug Fixes

  • Generate org.springframework.http.codec.multipart.Part and reactor.core.publisher.Flux<Part> when reactive=true with the spring-boot library.
  • Keep nullability and all non-file form fields unchanged; no changes for blocking or spring-declarative-http-interface outputs.
  • Added focused tests and regenerated Kotlin reactive samples.

• Migration

  • After regenerating a reactive Kotlin Spring Boot server, update handlers to accept Part (and Flux<Part> for arrays) instead of MultipartFile/Array<MultipartFile>.

^{Written for commit 805becf. Summary will update on new commits.}