fix(realtime): preserve twenty turns across recovery

[x-lixu]

Summary

• keep the foreground UI and Realtime recovery window aligned at twenty conversation turns (forty messages)
• add an injectable Realtime frontend seam for deterministic Gateway recovery tests
• verify content-safety rejection removes only the rejected turn while retaining earlier valid context

Validation

• node --test server/test/conversation-sync.test.mjs server/test/gateway-client-handshake.test.mjs server/test/session-conversation-history.test.mjs web/test/message-order.test.js