test(client): cover reconnect assistant backfill

[wizzoapp]

Summary

• add a client-runtime regression for the 7am failure mode: client receives an initial thread snapshot, disconnects, server canonical state gains a persisted assistant message, replacement session reconnects, and the reconnect snapshot renders the missed assistant message
• keeps the receive/backfill contract explicit; current implementation already resubscribes and applies snapshots on replacement sessions

Refs ADA-77.

Evidence

• vp test run src/state/threads-sync.test.ts --reporter verbose (8 passed)
• vp run --filter @t3tools/client-runtime typecheck
• Factory gate passed: static checks + clean Codex gpt-5.5 autoreview.

[adamfgr]

Auto-approved by wizzo-approve: all policy conditions verified against HEAD 3b7c197.