feat(bridgev2): receive room account data via double puppet /sync loop

[kalix127]

Draft: Manually tested on a real Synapse deployment with the WhatsApp bridge. Opening as draft for feedback on the approach.

Summary

• Add account data syncer that runs a /sync loop as the double-puppeted user to receive room account data changes (m.marked_unread, m.tag, com.beeper.mute)
• Synapse does not forward these events to appservices via the transaction API, so a separate sync loop is needed
• Content-based dedup prevents loops: when the bridge sets account data via double puppet, the resulting sync echo is identified by matching JSON content and skipped
• Add MarkAccountDataSent calls in intent methods (MarkRead, MarkUnread, TagRoom, MuteRoom) to track bridge-originated changes
• Add AccountDataSyncStopper interface so the sync loop is stopped when double puppeting is disabled

Manual testing results

Tested end-to-end with mautrix-whatsapp on Synapse with double puppeting configured. Account data events (m.marked_unread) are correctly received by the sync loop and dispatched to the bridge connector. Loop avoidance works, bridge-originated events are skipped by content comparison, and delayed Synapse re-deliveries (observed up to 3+ minutes late) are also handled correctly.

Test plan

• Mark a chat as unread from Element -> bridge connector receives the event and forwards to remote network
• Mark a chat as read from Element (open the chat) -> bridge connector receives read event
• Bridge sets m.marked_unread via double puppet -> sync echo is skipped (no infinite loop)
• Disable double puppeting -> account data sync loop stops cleanly
• Bridge restart -> sync loop resumes when double puppet is re-initialized

Fixes #479
Found while fixing mautrix/whatsapp#891