Andrey v markelov fix/python not enum validation

[wing328]

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

Fixes the Python generator so string properties constrained by a not composed schema that contains an enum now reject excluded values at validation time, instead of silently accepting them. Without this, tagged oneOf unions could accept values that belong on another branch.

Bug Fixes

• Emits a pydantic v2 field_validator that raises ValueError when a value matches the excluded enum set.
• Applies only to string enums in the not schema; pydantic v1 output and other property types are unchanged.
• Adds regression tests and regenerated petstore samples covering a TaggedUnion with Known and Other branches.

^{Written for commit c176f1d. Summary will update on new commits.}