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.

gateway/platforms/matrix.py

@@ -542,9 +542,9 @@ class MatrixAdapter(BasePlatformAdapter):
                 # no megolm sessions ever land. Setting _device_id here
                 # (in-memory; the on-disk row may not exist yet) makes
                 # the first put_account write the correct value.
+                # DeviceID is a NewType(str) so plain str works at runtime.
                 if client.device_id:
-                    from mautrix.types import DeviceID as _DeviceID
-                    await crypto_store.put_device_id(_DeviceID(client.device_id))
+                    await crypto_store.put_device_id(client.device_id)
 
                 crypto_state = _CryptoStateStore(state_store, self._joined_rooms)
                 olm = OlmMachine(client, crypto_store, crypto_state)