fix(gateway): use metadata thread_id for Mattermost root_id

[steveafrost]

Problem

Mattermost replies were not threading correctly. The root_id field must contain the thread root post ID, not the specific message being replied to.

Root Cause

The adapter was passing reply_to (the message_id) as root_id, causing each reply to start a new thread instead of continuing the existing one.

Solution

Use metadata["thread_id"] (the actual root post ID) when available, falling back to reply_to for backward compatibility.

Changes

• send_message(): Uses effective_reply_to = metadata.get("thread_id") or reply_to
• send_image_url(): Same pattern
• send_image_file(): Same pattern
• send_document(): Same pattern
• send_voice(): Same pattern
• send_video(): Same pattern

Testing

• Tested on self-hosted Mattermost instance
• Verified replies now correctly thread under the original message
• Backward compatible: falls back to reply_to if thread_id not provided

[alt-glitch]

Likely duplicate of #20874 which is the most comprehensive fix for this threading issue (also supersedes #6617, #12096, #12299, #18280). All address the same root cause: Mattermost adapter not using metadata.thread_id for root_id in outbound replies (see #12063 for the original issue report).

[steveafrost]

Closing as duplicate of #20874