fix(matrix): pass required args to MemoryCryptoStore for mautrix ≥0.21

MemoryCryptoStore.__init__() now requires account_id and pickle_key
positional arguments as of mautrix 0.21. The migration from matrix-nio
(commit 1850747) didn't account for this, causing E2EE initialization
to fail with:

  MemoryCryptoStore.__init__() missing 2 required positional arguments:
  'account_id' and 'pickle_key'

Pass self._user_id as account_id and derive pickle_key from the same
user_id:device_id pair already used for the on-disk HMAC signature.

Update the test stub to accept the new parameters.

Fixes #7803

gateway/platforms/matrix.py

@@ -352,7 +352,16 @@ class MatrixAdapter(BasePlatformAdapter):
                 from mautrix.crypto import OlmMachine
                 from mautrix.crypto.store import MemoryCryptoStore
 
-                crypto_store = MemoryCryptoStore()
+                # account_id and pickle_key are required by mautrix ≥0.21.
+                # Use the Matrix user ID as account_id for stable identity.
+                # pickle_key secures in-memory serialisation; derive from
+                # the same user_id:device_id pair used for the on-disk HMAC.
+                _acct_id = self._user_id or "hermes"
+                _pickle_key = f"{self._user_id}:{self._device_id}"
+                crypto_store = MemoryCryptoStore(
+                    account_id=_acct_id,
+                    pickle_key=_pickle_key,
+                )
 
                 # Restore persisted crypto state from a previous run.
                 # Uses HMAC to verify integrity before unpickling.