Add generateRootResources option for JavaJAXRSSpecServerCodegen

[rignicat]

Add generateRootResources (default enabled) to restore behaviour pre- #22169 which stopped generating root resources when interfaceonly was enabled.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
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Summary by cubic

Adds generateRootResources to JavaJAXRSSpecServerCodegen (default true) to generate the root resource and application classes when interfaceOnly is true, restoring the pre-#22169 behavior. This reintroduces RestApplication.java and RestResourceRoot.java in interface-only outputs; set the option to false to keep the newer no-root-resources behavior.

• Docs for jaxrs-spec and jaxrs-cxf-cdi list the new option.

Migration

• If you depend on interface-only outputs without root resources, pass --additional-properties=generateRootResources=false (or set generateRootResources: false in your config).

^{Written for commit 8d9f4d3. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 12 files

_{Re-trigger cubic}