Summary

• route send_message Feishu delivery through _send_feishu(..., media_files=...) when attachments are present
• attach media only on the final text chunk so chunked sends behave like other native-media platforms
• allow media-only Feishu sends to use the native Feishu path instead of the generic unsupported-media fallback
• update native-media support strings to include Feishu
• add regression coverage for chunked Feishu media routing and media-only Feishu delivery

Root cause

_send_feishu() already accepted and processed media_files, but _send_to_platform() sent Feishu through the generic non-media path and never passed attachments along. That caused MEDIA: attachments to be dropped for Feishu targets.

Test plan

• pytest -q tests/tools/test_send_message_tool.py -k "TestSendToPlatformFeishuMedia or TestSendToPlatformDiscordMedia::test_media_files_passed_on_last_chunk_only"
• pytest -q tests/gateway/test_feishu.py -k "send_image_file_uploads_image_and_sends_image_message or send_image_file_with_caption_uses_single_post_message"

Fixes #10136