feat(core): expose request body named examples to templates

[Mattias-Sehlstedt]

Adds so that any examples defined in a request body are properly attached to a CodegenParameter's examples so that they can be accessed within the mustache templates. This logic already exists for when reading a swagger-core OAS Parameter object, but have now also been extended to request bodies.

Fixes #23607

PR checklist

• Read the contribution guidelines.
• 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.
• 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

Expose all named request body examples to mustache via CodegenParameter.examples. Previously only a single example value was available; now the examples map from the request body’s first media type is attached while CodegenParameter.example remains unchanged.

• fromRequestBody now sets examples from the first content media type when present.
• Add getFirstContentMediaType and hasMediaType helpers; refactor getContentType, addConsumesInfo, getConsumesInfo, and addProducesInfo to use them (null-safety, no behavior change).
• Add tests and a spec path (/example6/multiple_examples) verifying multiple request body examples are accessible.
• No template migrations required; templates can iterate over request body examples.

^{Written for commit 321ce8d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}