[rust] fix #22147 handle rust generating null

[JensWalter]

• avoid using null values for further processing
• this resolves [BUG] [Rust] Cannot invoke "String.replaceAll(String, String)" because "name" is null #22147
• I added the API spec from the issue to the Test suite

PR checklist

• 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.
• 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.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Fix Rust codegen for anonymous oneOf variants that were generating null names, so models build correctly. Fixes #22147.

• Bug Fixes
  • Use "Variant{i}" when a oneOf schema has no title or type, avoiding null variant names.
  • Added a test with the issue’s API spec (rust-anonymous-oneof-model.yaml) to verify model generation.

^{Written for commit cfeea18. Summary will update on new commits.}

[wing328]

thanks for the PR

cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05) @jacob-pro (2022/10) @dsteeley (2025/07)

[JensWalter]

just a heads-up, it is not yet working, i found another issue in the generated Rust code. I will update the PR, if I find the root cause.

[wing328]

@JensWalter please take your time. Thanks for contributing a fix.