fix(matrix): add backoff for SyncError in sync loop (#3280)

When the homeserver returns an error response, matrix-nio parses it
as a SyncError return value rather than raising an exception. The sync
loop only had backoff in the except handler, so SyncError caused a
tight retry loop (~489 req/s) flooding logs and hammering the
homeserver. Check the return value and sleep 5s before retry.

Cherry-picked from PR #2937 by ticketclosed-wontfix.

Co-authored-by: ticketclosed-wontfix <ticketclosed-wontfix@users.noreply.github.com>

gateway/platforms/matrix.py

@@ -551,9 +551,20 @@ class MatrixAdapter(BasePlatformAdapter):
 
     async def _sync_loop(self) -> None:
         """Continuously sync with the homeserver."""
+        import nio
+
         while not self._closing:
             try:
-                await self._client.sync(timeout=30000)
+                resp = await self._client.sync(timeout=30000)
+                if isinstance(resp, nio.SyncError):
+                    if self._closing:
+                        return
+                    logger.warning(
+                        "Matrix: sync returned %s: %s — retrying in 5s",
+                        type(resp).__name__,
+                        getattr(resp, "message", resp),
+                    )
+                    await asyncio.sleep(5)
             except asyncio.CancelledError:
                 return
             except Exception as exc: