Fix Nullable imports for custom Spring generators

[jorgerod]

Fixes the bug reported in #21070: when using a custom generator whose name does not contain "spring", the Nullable imports were not added.
This change allows extending isSpringCodegen so custom generators can opt into Spring behavior and include the required imports.

Fixes #21070

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 Update swagger-parser to 2.0.1 #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. @cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)

---

Summary by cubic

Fixes missing Nullable imports for custom generators by allowing them to opt into Spring behavior. Changed SpringCodegen.isSpringCodegen() to protected so subclasses can enable Spring-style imports even if their name doesn’t contain “spring” (fixes #21070).

^{Written for commit b5948a6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[jorgerod]

Hi @wing328

Sorry for the inconvenience, but have you had a chance to take a look at it?

It would be very important to me that this PR be included in 7.21.0.

Thank you very much

[wing328]

just merged. thanks for the contribution