enhance python openapi generator

[padznich]

for Python 3.11+
anyOf / allOf Root model based
StrEnum, IntEnum

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• 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.
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@cbornet
@tomplus
@krjakbrjak
@fa0311
@multani

Fixes issues:

• [BUG][PYTHON] SDK generating on [ allOf, oneOf, anyOf ] not supported for items: #22260
• [BUG][PYTHON] SDK generating on anyOf is not supported within items #22261

---

Summary by cubic

Modernizes the Python generator for 3.11+ by adopting Pydantic v2 APIs and RootModel-based unions for anyOf/oneOf. Adds StrEnum/IntEnum, Literal-typed inline enums, simplifies serialization, and updates client/header handling for urllib3 v2.

• New Features

  • anyOf/oneOf now use RootModel[Union[...]] with optional discriminator.
  • Inline enum properties generate typing.Literal[...] types.
  • String and integer enums generate StrEnum/IntEnum.
  • Switch to Pydantic v2 methods (model_validate, model_dump, model_validate_json, model_dump_json).
  • Cleaner exceptions with unified body/data output.
  • ApiClient uses getheader/getheaders and model_validate for deserialization.

• Migration

  • Requires Python 3.11+; urllib3 >= 2.1.0.
  • Replace:
    • from_json → model_validate_json
    • to_json → model_dump_json(by_alias=True, exclude_unset=True)
    • from_dict → model_validate
    • to_dict → model_dump(by_alias=True)
  • anyOf/oneOf values are on .root.
  • ApiClient now calls klass.model_validate(data); response headers use getheaders() (list of pairs) instead of a mapping. Adjust code that reads headers accordingly.

^{Written for commit 82bb97c. Summary will update on new commits.}