fix: enable Feishu media delivery in send_message

Summary

• route Feishu through native send_message media delivery instead of dropping MEDIA: attachments
• preserve Feishu text chunking while attaching media only on the final chunk
• add regression coverage for Feishu text+media, media-only, and long-message chunking

Problem

The Feishu gateway adapter already supported native image/file sending, but the send_message tool did not expose that path. send_message parsed MEDIA:/path/to/file.png, yet _send_to_platform() only enabled native media delivery for Telegram and Weixin. For Feishu, media attachments were silently omitted even though the adapter could send them.

Root cause

tools/send_message_tool.py had a Feishu-specific helper (_send_feishu) that already accepted media_files, but _send_to_platform() routed Feishu through the generic non-media path.

Fix

• add a dedicated Feishu branch in _send_to_platform()
• preserve existing chunking behavior for long Feishu text messages
• attach media_files only on the final Feishu chunk
• update user-facing warning/error strings to reflect that native media delivery is now supported for Telegram, Feishu, and Weixin