fix(java): preserve user templateDir for feign-hc5 library

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fixes #23139
@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg
Can one of you take a look pls?

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Summary by cubic

Keep the user’s templateDir for Java feign-hc5 clients; only fall back to built-in feign templates when no custom dir is set. Fixes #23139.

• Bug Fixes
  • Guarded setTemplateDir(FEIGN) in JavaClientCodegen.processOpts with a CodegenConstants.TEMPLATE_DIR check.
  • Added tests to confirm defaulting to feign and preserving a custom templateDir.

^{Written for commit efc4b45. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

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