fix: feishu output issues (topic_group, UTF-8 truncation, ThreadID)

[haozicnm]

Summary

Fixes several issues found in the Feishu/Lark bot output code analysis.

Changes

🔴 Critical fixes

1. handleMessage now recognizes topic_group chat type (2f80b50)

   • Webhook path previously only handled "group", missing Feishu topic groups (topic_group)
   • This caused topic group messages to be treated as DM, bypassing RequireMention gating
   • Now matches the SDK path (handleSDKMessage) which already had this

2. UTF-8 safe tool output truncation (render.go)

   • Changed from byte-based len(output) > 500 to rune-based len([]rune(output)) > 500
   • Prevents garbled text when tool output contains multi-byte UTF-8 characters near the 500-byte boundary

🟡 Improvements

3. Added ThreadID field to feishuMsgEvent struct

   • Webhook push events include thread_id but it wasn't being parsed
   • handleMessage now extracts and propagates ThreadID to InboundMessage

4. Proper error handling in sendCard

   • json.Marshal error was silently ignored with _
   • Now properly returns the error with fmt.Errorf("feishu card marshal: %w", err)

Testing

• All existing tests pass (feishu: 9/9, bot: 20/20, full bot suite: 4/4 packages)
• Build verified with go build