fix(gateway): dedupe user turns on transient failure (#47237)

When the gateway persists a user message after a transient provider
failure (429/timeout/auth error), subsequent retries of the same
Telegram message could stack duplicate user turns in the transcript,
causing the agent to fall behind by 1-2 messages.

Add has_platform_message_id() to SessionDB (using the existing
idx_messages_platform_msg_id partial index) and a SessionStore wrapper.
The gateway's transient-failure path checks this before
append_to_transcript -- if the platform_message_id is already
persisted, the duplicate write is skipped.

Salvaged from #47869 by @davidgut1982. Adapted to current main which
has additional append sites and an existing content-based dedupe in
the exception handler path.

Closes #47237

tests/gateway/test_42039_duplicate_user_message.py

@@ -65,6 +65,9 @@ def _bootstrap(monkeypatch, tmp_path):
     )
     runner.session_store.load_transcript.return_value = []
     runner.session_store.append_to_transcript = MagicMock()
+    # Mock has_platform_message_id to return False so the dedupe guard
+    # (#47237) in gateway/run.py does not skip the append_to_transcript call.
+    runner.session_store.has_platform_message_id.return_value = False
     runner.session_store.update_session = MagicMock()
 
     monkeypatch.setattr(gateway_run, "_hermes_home", tmp_path)