based on #22599 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

Add @nullable to the toIndentedString parameter in generated Java Spring models to align with nullability handling and silence static analysis warnings. Updated tests and regenerated samples; no runtime behavior change.

• Bug Fixes
  • Add {{>nullableAnnotation}} to toIndentedString parameter in JavaSpring/pojo.mustache.
  • Extend SpringCodegenTest to assert the annotated signature.
  • Regenerate affected Spring samples (cloud, http-interface, reactive) to include the change.

^{Written for commit 20383b7. Summary will update on new commits.}