fix: guard mautrix imports for gateway-safe fallback + fix test isolation

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).

tests/gateway/test_matrix.py

@@ -601,6 +601,40 @@ class TestMatrixDisplayName:
 # Requirements check
 # ---------------------------------------------------------------------------
 
+class TestMatrixModuleImport:
+    def test_module_importable_without_mautrix(self):
+        """gateway.platforms.matrix must be importable even when mautrix is
+        not installed — otherwise the gateway crashes for ALL platforms.
+
+        This test uses a subprocess to avoid polluting the current process's
+        sys.modules (reimporting a module creates a second module object whose
+        classes don't share globals with the original — breaking patch.object
+        in subsequent tests).
+        """
+        import subprocess
+        result = subprocess.run(
+            [sys.executable, "-c", (
+                "import sys\n"
+                "# Block mautrix completely\n"
+                "class _Blocker:\n"
+                "    def find_module(self, name, path=None):\n"
+                "        if name.startswith('mautrix'): return self\n"
+                "    def load_module(self, name):\n"
+                "        raise ImportError(f'blocked: {name}')\n"
+                "sys.meta_path.insert(0, _Blocker())\n"
+                "for k in list(sys.modules):\n"
+                "    if k.startswith('mautrix'): del sys.modules[k]\n"
+                "from gateway.platforms.matrix import check_matrix_requirements\n"
+                "assert not check_matrix_requirements()\n"
+                "print('OK')\n"
+            )],
+            capture_output=True, text=True, timeout=10,
+        )
+        assert result.returncode == 0, (
+            f"Subprocess failed:\nstdout: {result.stdout}\nstderr: {result.stderr}"
+        )
+
+
 class TestMatrixRequirements:
     def test_check_requirements_with_token(self, monkeypatch):
         monkeypatch.setenv("MATRIX_ACCESS_TOKEN", "syt_test")
@@ -738,7 +772,7 @@ class TestMatrixE2EEHardFail:
 
     @pytest.mark.asyncio
     async def test_connect_fails_when_encryption_true_but_no_e2ee_deps(self):
-        from gateway.platforms.matrix import MatrixAdapter
+        from gateway.platforms.matrix import MatrixAdapter, _check_e2ee_deps
 
         config = PlatformConfig(
             enabled=True,
@@ -768,7 +802,8 @@ class TestMatrixE2EEHardFail:
         from gateway.platforms import matrix as matrix_mod
         with patch.object(matrix_mod, "_check_e2ee_deps", return_value=False):
             with patch.dict("sys.modules", fake_mautrix_mods):
-                result = await adapter.connect()
+                with patch.object(adapter, "_sync_loop", AsyncMock(return_value=None)):
+                    result = await adapter.connect()
 
         assert result is False