[swarnendusahu10-dotcom]

Description

Closes #1350

Implements the thread message event listener system in EmbeddedChatApi.ts, fulfilling the long-standing TODO at line 239 which marked this as pending after thread UI implementation.

Problem

When a user sends a reply inside an open thread, the message arrives over the DDP room-messages stream carrying a tmid (thread message ID) field pointing to its parent. Previously, no code dispatched these messages to thread-specific listeners — the onThreadMessageCallbacks array was declared but never populated or fired. As a result, the thread panel could not receive real-time updates and users had to manually refresh to see new replies.

Changes Made

File modified: packages/api/src/EmbeddedChatApi.ts (1 file, 27 lines added, 1 deleted)

• Added onThreadMessageCallbacks property (typed identically to onMessageCallbacks).
• Initialized to [] in the constructor.
• In the room-messages DDP stream handler: when a normalized message has a truthy tmid, all registered thread listeners are called.
• Same tmid guard added to the notify-user/message stream (covers action-block messages sent inside threads).
• Added addThreadMessageListener() and removeThreadMessageListener() following the exact dedup/filter pattern used by every other listener type in this class.
• Removed the resolved TODO comment.

Verification

• All 10 edge-case scenarios pass (normal vs thread messages, dedup, removal, null safety, falsy tmid, listener leak prevention).
• Rollup build of packages/api succeeds with no TypeScript errors.
• Full packages/react build also succeeds with no warnings attributable to this change.
• Zero changes to any React component, store, or other file — no domino effects.