[python] Implement branch CRUD on FileSystemCatalog

[TheR1sing3un]

Purpose

Override the five branch CRUD methods on FileSystemCatalog so Python users on the local-catalog path get the same typed behavior as RESTCatalog (added in #7747). Sister PR to #7751 (FileSystemCatalog Tag CRUD); follow-up to #7747.

FileSystemCatalog previously inherited the Catalog ABC NotImplementedError stubs because the original PR (#7747) did not port the manager-layer wiring. The Python FileSystemBranchManager already exists at pypaimon/branch/filesystem_branch_manager.py and is reachable via FileStoreTable.branch_manager(), so this PR is a thin wrapper: each catalog method delegates to the manager and translates ValueError messages back into the typed catalog exception hierarchy (BranchAlreadyExistException, BranchNotExistException, TagNotExistException).

In scope

• 5 method overrides on FileSystemCatalog: create_branch / drop_branch / rename_branch / fast_forward / list_branches.
• New end-to-end test suite tests/filesystem_catalog_branch_test.py covering the same matrix as RESTCatalogBranchCRUDTest from [python] Add branch CRUD REST implementation to RESTCatalog #7747 (create / list / rename / drop / fast_forward — happy paths plus all typed-exception paths).
• Removal of the now-stale FilesystemCatalogBranchInheritsNotImplementedTest class from rest/rest_branch_test.py (it asserted NotImplementedError, which no longer holds).

Out of scope

• The FileSystemBranchManager "from-tag" path has a pre-existing bug: _copy_with_branch returns a SnapshotManager that doesn't switch to the new branch's path, so copy_file(src, dst) ends up with src == dst and raises SameFileError. That's a manager-level fix. The catalog-layer error translation for the from-tag path is still covered by test_create_branch_from_nonexistent_tag_raises. Two corresponding happy-path tests (test_create_branch_from_existing_tag and test_fast_forward_happy) are intentionally omitted with NOTE comments pointing at the bug.
• ignore_if_exists parameter on create_branch — Java Catalog.createBranch does not expose it, and the manager-layer default (False) is sufficient.

Tests

Local run from paimon-python/:

pytest pypaimon/tests/filesystem_catalog_branch_test.py -v   # 13 passed
pytest pypaimon/tests/rest/rest_branch_test.py \
       pypaimon/tests/api/test_branch_dto_serde.py \
       pypaimon/tests/filesystem_catalog_tag_test.py \
       pypaimon/tests/branch_manager_test.py -v               # 59 passed
flake8 --config=dev/cfg.ini <touched files>                   # clean

Anti-divergence checklist

• Method signatures match the Catalog ABC (identifier: Union[str, Identifier], tag_name=None).
• Typed exceptions match those raised by RESTCatalog (BranchAlreadyExistException / BranchNotExistException / TagNotExistException / TableNotExistException).
• list_branches returns List[str] (not PagedList), matching the strict Java alignment from [python] Add branch CRUD REST implementation to RESTCatalog #7747.
• Substring matches verified against pypaimon/branch/filesystem_branch_manager.py master HEAD.

Generative AI disclosure

Drafted with assistance from a generative AI tool. All code, tests, and exception-translation logic were reviewed and validated by the contributor.

[JingsongLi]

+1