Follow-up fixes for the matrix-nio → mautrix migration:
1. Module-level mautrix.types import now wrapped in try/except with
proper stub classes. Without this, importing gateway.platforms.matrix
crashes the entire gateway when mautrix isn't installed — even for
users who don't use Matrix. The stubs mirror mautrix's real attribute
names so tests that exercise adapter methods (send, reactions, etc.)
work without the real SDK.
2. Removed _ensure_mautrix_mock() from test_matrix_mention.py — it
permanently installed MagicMock modules in sys.modules via setdefault(),
polluting later tests in the suite. No longer needed since the module
imports cleanly without mautrix.
3. Fixed thread persistence tests to use direct class reference in
monkeypatch.setattr() instead of string-based paths, which broke
when the module was reimported by other tests.
4. Moved the module-importability test to a subprocess to prevent it
from polluting sys.modules (reimporting creates a second module object
with different __dict__, breaking patch.object in subsequent tests).
be9198f1e1