Define uses_psycopg2_extras on both Postgres engines

The driver-agnostic base declares it and the executescript path reads it, but neither concrete engine set it, so booting on the Rust engine failed with an AttributeError in schema preparation.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULdieJyQoHNzeHq42H2M1p

synapse/storage/engines/postgres.py

@@ -43,6 +43,10 @@ class Psycopg2Engine(
 ):
     """The Postgres backend that talks to the database via psycopg2."""
 
+    # psycopg2's cursor is a real psycopg2 cursor, so the `psycopg2.extras`
+    # helpers can be used on it directly.
+    uses_psycopg2_extras: bool = True
+
     def __init__(self, database_config: Mapping[str, Any]):
         super().__init__(psycopg2, database_config)
         psycopg2.extensions.register_type(psycopg2.extensions.UNICODE)

synapse/storage/engines/postgres_rust.py

@@ -57,6 +57,11 @@ _RETRYABLE_PGCODES = ("40001", "40P01")
 class RustPostgresEngine(PostgresEngine[Connection, Cursor]):
     """A :class:`PostgresEngine` that talks to the Rust backend's shim."""
 
+    # The shim cursor is not a psycopg2 cursor, so the `psycopg2.extras`
+    # helpers can't be used on it; `LoggingTransaction.executescript` takes the
+    # shim's own multi-statement path instead.
+    uses_psycopg2_extras: bool = False
+
     # SQL isolation-level names for each `IsolationLevel`. The shim has no
     # psycopg2-style `set_isolation_level`, so a per-transaction override is
     # applied as a `SET SESSION CHARACTERISTICS` statement (see