test(state): make the state manager tests deterministic and say what they test

[allen0099]

Summary

Closes #186 (part of #123). Test-only change to tests/state/test_manager.py.

• test_consume_expired_state replaced.
  • The old test slept past a 1 s TTL and accepted either InvalidStateError or StateExpiredError. Its docstring said Redis keeps the data after the TTL, which it doesn't.
  • The new test_consume_state_after_backend_ttl_is_invalid moves the memory backend's clock past the TTL, with no sleep, and expects exactly InvalidStateError("Invalid or expired state").
  • The other case, where the entry still exists but expires_at has passed, raises StateExpiredError. test_consume_state_past_wall_clock_expiry_removes_the_entry already covers it deterministically; the new docstring points there.
• test_get_metadata_with_missing_expiry and test_validate_state_with_missing_expiry now test what their names say.
  • They store an entry with no expires_at.
  • They expect None and False, the documented "otherwise" result of get_state_metadata / validate_state. expires_at is a required StateData field, so the entry fails to decode.
  • They also check that peeking leaves the entry in place.
  • Before, they stored a valid expires_at and repeated the happy path.
• Removed the unused memory_backend_for_state and redis_backend_for_state fixtures.
• Dropped the try/finally BackendProxy resets. The autouse setup_default_backend (in tests/conftest.py) sets a fresh MemoryBackend on BackendProxy before every test. The other proxies are reset by reset_proxy_singletons. test_state_manager_accepts_explicit_backend now uses the memory_backend fixture for cleanup.

No library changes. The behaviour for a missing expires_at matches the docs, so no library bug was found.

Verification

• uv run pytest without live servers: 814 passed, 190 skipped. Before: 814 passed, 191 skipped; the removed Redis variant of the flaky test accounts for the difference. Runtime is about 24.6 s, down from 27.4 s.
• Coverage of fastapi_cachex/state/: 100% before and after (statements and branches); manager.py is 112/112 statements and 16/16 branches.
• Mutation checks (full suite each time):
  • Memory get_and_delete returning expired entries: fails the new test_consume_state_after_backend_ttl_is_invalid, plus the existing memory backend test for that behaviour.
  • StateData.expires_at given a default, so a missing expires_at decodes: fails both rewritten missing-expiry tests, plus the existing test_consume_state_with_missing_expiry.
• pre-commit --all-files: passed. uv lock --check ok.

CHANGELOG

None; test-only change.