fix: route active thread replies to the last active bot

[LIU9293]

Summary

• Scope thread active-state routing to the current bot identity by tracking lastActivityBotId in sessions.
• Remove the multi-bot hard mention requirement so active replies without @bot follow the last active bot in the thread.
• Update Slack/Lark/Discord inbound paths plus shared inbound decision types/tests to match the new routing behavior.

Testing

• bun test packages/ims/slack/message-router.test.ts
• bun test packages/ims/shared/incoming-message-processor.test.ts
• bun test packages/core/test/session-retention.test.ts