fix(feishu): route thread replies via Lark reply endpoint

Bot responses to messages inside Lark topic threads were dropped
outside the thread because outbound Send always used the new-message
endpoint. This change:

- Adds LarkClient.ReplyMessage() that POSTs to
  /open-apis/im/v1/messages/{id}/reply with reply_in_thread=true
- Parses thread_id from im.message.receive_v1 events (distinct from
  root_id which fires on any quote reply) and stamps
  feishu_reply_target_id into the message metadata
- Propagates the key through cmd/gateway_consumer_normal.go and the
  new package-level routingMetaKeys var in internal/channels/events.go
  so block replies and retry notifications also land in thread
- Routes sendText, sendMarkdownCard, sendImage, sendFile, and
  sendMediaAttachment via a new deliverMessage helper that falls back
  to SendMessage with a warning log on reply endpoint errors (e.g.
  thread root deleted)
- Adds 10 unit tests covering routing, fallback, content
  double-encoding, and the thread_id gate that prevents plain quote
  replies from being silently promoted to threads

Closes #818

cmd/gateway_consumer_normal.go

@@ -220,7 +220,7 @@ func processNormalMessage(
 			outMeta["reply_to_message_id"] = mid
 		}
 	}
-	for _, k := range []string{tools.MetaMessageThreadID, "local_key", "placeholder_key", "group_id"} {
+	for _, k := range []string{tools.MetaMessageThreadID, "local_key", "placeholder_key", "group_id", "feishu_reply_target_id"} {
 		if v := msg.Metadata[k]; v != "" {
 			outMeta[k] = v
 		}