[BUG] [KOTLIN-SPRING] @HttpExchange in declarative interface does not support property placeholders

[Picazsoo]

See #22879 for description of the issue. Basically the gist is:

• get rid of placeholders in the class-level @HttpExchange annotation and replace it with the generated BASE_PATH property.
• allow skipping of this class-level @HttpExchange annotation via requestMappingMode property. Hence allowing entirely custom base path if so desired
• still make the original BASE_PATH available as a const val in companion object as it can be also set programatically as the base path into the webclient/restclient and this client can be reused in more places.

To make it clearer - as of now, generating with requestMappingMode = api_interface basically produces non-working code unless extra steps are taken when instantiating the interface by adding StringValueResolver via HttpServiceProxyFactory.builder()

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Summary by cubic

Fixes placeholder usage in Kotlin Spring @HttpExchange by switching to a generated BASE_PATH constant and optional interface-level mapping. This makes generated clients work with HttpServiceProxyFactory and avoids unresolved placeholders. Fixes #22879.

• Bug Fixes
  • Generate BASE_PATH in each API interface and use it in @HttpExchange when requestMappingMode=api_interface.
  • Remove "${...}" placeholders from the template; import Companion.BASE_PATH where used.
  • Set requestMappingMode: "api_interface" for the wrapped sample; other samples keep only BASE_PATH with no interface annotation.
  • Update tests to assert the new import/annotation and to verify no annotation when requestMappingMode="none"; refresh samples accordingly.

^{Written for commit 121956c. Summary will update on new commits.}

[cubic-dev-ai]

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