test(_ext[sphinx_pytest_fixtures]): add Sphinx integration tests

why: Unit tests verify individual functions but cannot catch issues in
the full Sphinx rendering pipeline — directive registration, doctree
transforms, cross-reference resolution, intersphinx link generation,
and CSS inclusion all require end-to-end Sphinx builds.
what:
- 1,853 lines of pytest integration tests (marked @pytest.mark.integration)
  using Sphinx's make_app test helper, covering:
  - AutofixturesDirective bulk discovery and source-order rendering
  - AutofixtureIndexDirective table structure and linked return types
  - PyFixtureDirective with :scope:, :kind:, :deprecated: options
  - :fixture: cross-reference resolution with short-name fallback
  - Badge rendering for all scope/kind/state permutations
  - Parametrized values as inline and enumerated lists
  - Teardown summary injection into yield-fixture callouts
  - "Used by" cross-document link generation
  - External URL rendering for dependency links
  - :usage: none suppression of auto-generated snippets
  - Excluded fixtures absent from index table
  - Scope/kind pair-index entry structure
  - SPF003 warning for yield fixtures via autodoc
  - Async fixture callout rendering
  - Text builder compatibility (no crash on non-HTML)

Author: tony