[php-nextgen] Remove request body handling if no request body exists

[coffeemakr]

• Remove request processing if there is no body (unused code)
• Change imports of FormProcessor and MultipartStream so no unused imports exist

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

Remove request body/form handling and related variables/imports from generated php-nextgen APIs when operations have no body. This cleans up dead code and avoids unused imports in generated clients.

• Refactors
  • Update api.mustache to emit body/form blocks only when needed; remove unused FormDataProcessor import and inline MultipartStream usage.
  • Regenerate PHP samples to drop empty $formParams, $queryParams, and body-processing code where not applicable.
  • Add test and fixture to ensure no form/body blocks are generated without body/form params.

^{Written for commit c8962bd. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 22 files

_{Re-trigger cubic}