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

[ticketclosed-wontfix]

Summary

When the Matrix 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 logic in the except handler, so SyncError responses caused an immediate tight retry loop (~489 req/s) that flooded logs with thousands of 'next_batch' is a required property warnings per second and hammered the homeserver.

Fix

Check the sync() return value for nio.SyncError and sleep 5s before retrying, matching the existing exception-handling backoff.

Reproduction

1. Run Hermes gateway with a Conduwuit homeserver
2. Trigger a transient sync error (e.g., restart Conduwuit mid-sync)
3. Observe logs filling at ~489 lines/second with WARNING nio.responses: Error validating response: 'next_batch' is a required property

Testing

• Verified the sync loop processes normal SyncResponse objects unchanged
• Verified SyncError responses now trigger a 5s backoff with a descriptive warning
• self._closing guard prevents 5s shutdown delay, matching existing pattern at line 561

[teknium1]

Merged via PR #3280. Your commit was cherry-picked onto current main with authorship preserved. Clean fix, thanks!