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

internal/channels/feishu/bot_parse.go

@@ -16,6 +16,7 @@ func (c *Channel) parseMessageEvent(event *MessageEvent) *messageContext {
 	contentType := msg.MessageType
 	rootID := msg.RootID
 	parentID := msg.ParentID
+	threadID := msg.ThreadID
 
 	senderID := ""
 	if sender != nil {
@@ -58,6 +59,7 @@ func (c *Channel) parseMessageEvent(event *MessageEvent) *messageContext {
 		MentionedBot: mentionedBot,
 		RootID:       rootID,
 		ParentID:     parentID,
+		ThreadID:     threadID,
 		Mentions:     mentions,
 	}
 }