feat: explore Python API client codegen

[EastSun5566]

Experiment

Try @hey-api/openapi-python with the same vendored v1 OpenAPI spec used by the Node.js API client. This draft PR is stacked on #62; nothing is published.

• Generate all 59 raw operations with flat parameters and commit the output under python/src/hackmd_api/generated/. Align codegen / check:generated with Node.js.
• Add a small API wrapper for profile, teams, history, personal notes/images, personal folders/order, and team note detail. Reuse generated Pydantic models, with ETag/304 handling, bounded retries for reads/PUT/DELETE, and HTTP errors. POST/PATCH are not retried; api.raw keeps the original HTTPX responses.
• Pin dependencies and temporarily patch hey-api/hey-api#4441 (enum names) and hey-api/hey-api#4290 (flat request parameters/JSON bodies). Additional local changes provide per-request headers/multipart overrides and omit unset model fields while preserving explicit nulls. Remove each patch only after a release provides equivalent behavior; those two PRs alone do not cover all additions.

Result

Synced with the latest #62 contract and regenerated the Python client. Generated-file consistency checks, compilation, and all 19 offline HTTPX tests pass locally on Python 3.13. Tests cover authentication/base URLs, paths/query/JSON, multipart bytes, ETag, retry isolation, errors, nullable team owners, and omitted/string/null note descriptions. Note GET descriptions remain strings.

The previous production read-only live run passed all 5 read-only tests; the 2 mutation workflows were skipped. The new Python suite covers the existing Node live scenarios plus 200 → 304 → changed 200, but Python's full mutation run is still pending explicit authorization. Cleanup tracks created resources and restores folder order; notes go to trash, and uploaded image removal is not guaranteed.

Python CI runs generated-file checks, compilation, and offline tests on Python 3.13 for relevant PRs, including this stack. It uses no API credentials and runs no live tests.

This remains an experiment: raw multipart uploads still need request overrides (tracked in hey-api/hey-api#4443), flat optional arguments do not distinguish omitted values from None, nullable flat parameters may be typed as Any, and generated nullable fields do not fully enforce required-field presence. Use Pydantic bodies through the wrapper or request_overrides to preserve explicit nulls. Grouped parameters, async wrappers, packaging/publication, and custom wrappers for all operations remain out of scope.