#24043 update python-fastapi multipart dep to newer version

to close https://github.com/OpenAPITools/openapi-generator/pull/24040

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.
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Summary by cubic

Updated python-multipart from 0.0.27 to 0.0.31 in the Python FastAPI generator template and the petstore sample. This keeps the dependency current and aligns generated projects with the latest package version.