Fix async fixture autouse=True support (issue #123)

[VedantBhayani]

Fixes issue #123 - async fixtures with autouse=True were not being automatically applied to tests.

Changes

• pytest_trio/plugin.py: Detect autouse fixtures via fixturedef._autouse, store on test item, include in nursery execution, register dependencies
• pytest_trio/_tests/test_autouse.py: New test file with 6 comprehensive tests covering:
  • autouse=True with @pytest.mark.trio
  • autouse=True in trio_mode = true
  • Multiple autouse async fixtures
  • Autouse async fixture with dependency
  • Autouse async yield fixture
  • Autouse fixture with regular test dependency

Testing

• All 63 tests pass (including 6 new autouse tests)
• 2 pre-existing xfails unchanged
• Works with both @pytest.mark.trio and trio_mode = true configurations

Fixes #123.