#25125 Add tests for not in python client

a follow up pr to https://github.com/OpenAPITools/openapi-generator/pull/25070

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.

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Summary by cubic

Adds test schemas for the OpenAPI not keyword to the Python client test spec, generating new Known, Other, and TaggedUnion models with unit tests across all Python sample variants.

• Known allows only the known kind value, Other rejects known and reserved via not, and TaggedUnion combines them in a oneOf.
• Regenerates samples for aiohttp, httpx, httpx2, lazyImports, and pydantic-v1 variants so CI sample checks pass.