Add configurable generic return type wrapper for JAX-RS codegen

[EricWittmann]

Summary

• Adds a genericReturnType setting to JaxRsProjectSettings that wraps all generated JAX-RS return types in a user-specified generic class (e.g. org.jboss.resteasy.reactive.RestResponse)
• Enables type-safe HTTP status code control: Beer getBeer(...) becomes RestResponse<Beer> getBeer(...), void methods become RestResponse<Void>, and reactive mode produces CompletionStage<RestResponse<T>>
• Based on the approach from stale PR Add option to wrap return type in a generic class #295, reimplemented against the current codebase

Changes

• JaxRsProjectSettings.java — new genericReturnType field (default null = no wrapping)
• OpenApi2JaxRs.java — new generateGenericReturnType() method, inserted into the return type chain between type generation and reactive wrapping; updated setVoidReturnType() to handle the wrapper
• OpenApi2JaxRsTest.java — 2 new test methods (generic-only and generic+reactive), new doFullTest overload
• Test fixtures — 2 new expected output directories

Test plan

• All 42 existing + new tests pass (mvn test -pl core)
• Verified generated output: RestResponse<Beer>, RestResponse<Void>, CompletionStage<RestResponse<T>>
• Quarkus codegen inherits the feature automatically (no changes needed)
• Verify CI passes

Resolves #430

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