Add automatic ChunkID generation for messages in Send method

- Implemented logic to auto-generate ChunkID for messages when not provided, enhancing message tracking.
- Utilized IDGenerator if available; otherwise, fallback to message.GenerateNanoID for unique identification.
- Improved message handling consistency by ensuring ChunkID is always set for non-event messages.

agent/context/output.go

@@ -39,6 +39,15 @@ func (ctx *Context) Send(msg *message.Message) error {
 		}
 	}
 
+	// === Auto-generate ChunkID (always) ===
+	if msg.ChunkID == "" && !isEventMessage {
+		if ctx.IDGenerator != nil {
+			msg.ChunkID = ctx.IDGenerator.GenerateChunkID()
+		} else {
+			msg.ChunkID = message.GenerateNanoID()
+		}
+	}
+
 	// === Non-delta operations: New message logic ===
 	if !msg.Delta && !isEventMessage {
 		// Auto-set ThreadID for non-root Stack (nested agent calls)