Clean up interfaceOnly option

[wing328]

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Summary by cubic

Standardized the interfaceOnly option across server generators by centralizing its key/description and updating docs/tests. Also clarified Go Gin help text to state it generates stubs instead of implementation files.

• Refactors
  • Added CodegenConstants.INTERFACE_ONLY and INTERFACE_ONLY_DESC.
  • Switched GoGinServerCodegen, JavaDubboServerCodegen, JavaJAXRSSpecServerCodegen, JavaVertXWebServerCodegen, KotlinServerCodegen, KotlinSpringServerCodegen, and SpringCodegen to the shared key; removed local duplicates.
  • Unified help text to “Whether to generate only API interface stubs without the server files.”; for Go Gin, clarified wording to “Whether to generate only API interface stubs instead of the API implementation files.” and updated docs/generators/go-gin-server.md.
  • Updated tests to reference the shared constant and cleaned up minor assertions.

^{Written for commit 5e1d404. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 15 files

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