test(_ext[sphinx_pytest_fixtures]): add unit tests for extension submodules

why: Each extension submodule (_detection, _metadata, _store, _badges,
_directives, _validation, _index) has branching logic and edge cases
that must be tested in isolation without requiring full Sphinx builds.
what:
- 1,385 lines of pytest tests covering:
  - _is_pytest_fixture positive/negative detection
  - _get_user_deps filtering of hidden and builtin fixtures
  - _classify_deps partitioning into internal, builtin, external
  - _get_return_annotation with typing generics and forward refs
  - _is_factory heuristic for type[X] and Callable returns
  - _build_usage_snippet for resource, factory, override_hook kinds
  - _build_badge_group_node for scope/kind/autouse/deprecated badges
  - _qualify_forward_ref with TYPE_CHECKING block resolution
  - _extract_summary sentence boundary and EOF handling
  - _extract_teardown_summary from Teardown docstring sections
  - SPF001–SPF009 validation warnings and error-level lint
  - FixtureMeta.param_reprs wiring from :params: directive option
  - Store env_version guard and docname collision detection
  - CSS class contract tests and tabindex accessibility
  - Index table Flags column badge rendering

Author: tony