[python] Implement tag CRUD on FileSystemCatalog

[TheR1sing3un]

Purpose

PR #7746 added abstract tag stubs to Catalog and a complete RESTCatalog implementation, but left FileSystemCatalog inheriting NotImplementedError. The merge note said implementing filesystem tag CRUD required porting a Python TagManager. As it turns out, pypaimon/tag/tag_manager.py::TagManager and FileStoreTable.create_tag / delete_tag / list_tags / tag_manager() already exist on master, so this PR is a thin catalog-layer wrapper that delegates to those helpers and translates ValueError / return-value-False into the typed catalog exceptions used by the rest of the API.

Linked issue

Follow-up to #7746 — closes the FileSystemCatalog gap noted in that PR's "Out of scope" section.

Tests

• pypaimon/tests/filesystem_catalog_tag_test.py (12 new cases) — mirrors RESTCatalogTagCRUDTest on the local filesystem path:
  • test_create_and_get_tag, test_create_tag_with_snapshot_id
  • test_create_tag_already_exists_raises, test_create_tag_already_exists_ignore
  • test_create_tag_with_time_retained_raises_not_implemented (filesystem-only)
  • test_create_tag_table_not_exists
  • test_get_tag_not_exists
  • test_delete_tag_happy, test_delete_tag_not_exists
  • test_list_tags_paged_basic (paging via max_results + next_page_token)
  • test_list_tags_paged_with_prefix, test_list_tags_paged_empty_table
• pypaimon/tests/rest/rest_tag_test.py — removed the now-stale FilesystemCatalogTagInheritsNotImplementedTest class (its assertions no longer hold; new behavior is covered by the new file). Cleaned up unused imports left behind.
• Regression: pypaimon/tests/api/test_tag_dto_serde.py (8 cases) and the REST tag test matrix (11 cases) all still pass.
• All edited files pass flake8 --config=dev/cfg.ini.

API and format

No new public Catalog method or wire format. This PR only overrides the four existing abstract stubs in FileSystemCatalog:

Method	Delegate / behavior
create_tag	table.create_tag(tag_name, snapshot_id, ignore_if_exists). ValueError("...already exists.") → TagAlreadyExistException (only when not ignore_if_exists).
delete_tag	table.delete_tag(tag_name) -> bool. False → TagNotExistException.
get_tag	table.tag_manager().get(tag_name) -> Optional[Tag]. None → TagNotExistException. Result wrapped in GetTagResponse with snapshot=tag.trim_to_snapshot().
list_tags_paged	table.list_tags() -> List[str]. Client-side lexicographic sort + prefix filter + page_token continuation cursor (no Java equivalent of paged listing exists in TagManager itself).

Documentation

n/a — this is a parity fix for an existing public API.

Out of scope (separate PRs)

• time_retained real support — Python's Tag dataclass currently inherits only Snapshot fields and has no place to store tag_create_time / tag_time_retained. This PR rejects time_retained != None with NotImplementedError rather than silently dropping it (which would mislead callers about TTL effects). Extending Tag + TagManager.create_tag to carry these fields requires a coordinated change to the on-disk tag JSON format and is tracked as a follow-up.
• GetTagResponse.tag_create_time — for the same reason, this catalog returns None for tag_create_time and tag_time_retained. Cross-FileIO mtime extraction is not reliable (different FileIO implementations expose different metadata).
• Branch CRUD on FileSystemCatalog — branch CRUD requires the abstract rename_branch stub and BranchAlreadyExistException handling introduced by [python] Add branch CRUD REST implementation to RESTCatalog #7747 (still under review). The sister PR for FileSystemCatalog branch CRUD will follow once [python] Add branch CRUD REST implementation to RESTCatalog #7747 lands.
• Tag callbacks — Java has them; Python doesn't, and they're not needed for filesystem semantics.

Verification

# Targeted
pytest pypaimon/tests/filesystem_catalog_tag_test.py -v

# Regression
pytest pypaimon/tests/rest/rest_tag_test.py pypaimon/tests/api/test_tag_dto_serde.py -v

# Master-vs-fix
git stash
python -c "from pypaimon.catalog.filesystem_catalog import FileSystemCatalog; \
           print('create_tag in __dict__:', 'create_tag' in FileSystemCatalog.__dict__)"
# Expected: False (master inherits NotImplementedError)
git stash pop
python -c "..."  # Expected: True (PR adds the override)

# Lint
flake8 --config=dev/cfg.ini pypaimon/catalog/filesystem_catalog.py \
                            pypaimon/tests/filesystem_catalog_tag_test.py \
                            pypaimon/tests/rest/rest_tag_test.py

Generative AI disclosure

Implementation, tests, and PR description were drafted with the assistance of Claude (Anthropic). Claude was supplied with the existing pypaimon helpers (TagManager, FileStoreTable.create_tag / delete_tag / list_tags, RESTCatalog.create_tag's exception mapping pattern from #7746) and the REST tag test matrix; the design plan and code were reviewed and integrated by the author.

[JingsongLi]

+1