based on #22603 with resolved merge conflicts

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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Summary by cubic

Adds a null check for HttpServletResponse in ApiUtil.setExampleResponse to prevent NullPointerException in generated Spring projects. Updates all Spring samples to reflect the change.

• Bug Fixes
  • Guard req.getNativeResponse(HttpServletResponse.class) with a null check and no-op when null.
  • Updated JavaSpring apiUtil.mustache and regenerated Spring samples.

^{Written for commit 7d0994f. Summary will update on new commits.}