fix(matrix): drop needless DeviceID import + mock put_device_id in tests

Two adjustments to make CI pass:

- In gateway/platforms/matrix.py: `DeviceID` is `NewType("DeviceID", str)`,
  so passing `client.device_id` directly (already a str) works identically
  at runtime. The explicit import was cosmetic and tripped CI environments
  where `mautrix.types` doesn't re-export DeviceID at the expected path
  ("cannot import name 'DeviceID' from 'mautrix.types' (unknown location)").

- In tests/gateway/test_matrix.py: add `put_device_id` to the hand-written
  `PgCryptoStore` fake so the three encryption-path tests
  (test_connect_with_access_token_and_encryption,
  test_connect_uses_configured_device_id_over_whoami,
  test_connect_registers_encrypted_event_handler_when_encryption_on) can
  exercise the new crypto-store binding without AttributeError.

tests/gateway/test_matrix.py

@@ -197,10 +197,14 @@ def _make_fake_mautrix():
             self.account_id = account_id
             self.pickle_key = pickle_key
             self.db = db
+            self._device_id = ""
 
         async def open(self):
             pass
 
+        async def put_device_id(self, device_id):
+            self._device_id = device_id
+
     mautrix_crypto_store_asyncpg.PgCryptoStore = PgCryptoStore
 
     # --- mautrix.util ---