Fix Mattermost @mention detection in group channels

[anathn]

Fix Mattermost @mention detection in group channels

Background

The Mattermost channel adapter was parsing incoming WebSocket events but always returned an empty metadata map — meaning there was no way to know if the bot had been directly @mentioned in a group channel.

Mattermost includes a mentions JSON array in the event payload listing the user IDs of anyone mentioned in that message. The fix reads that array and sets was_mentioned: true in the message metadata when the bot's own user ID appears in it. This only applies to group channels since DMs are always directed at the bot.

Summary

• Parse the mentions JSON array from Mattermost WebSocket event data
• Set was_mentioned: true in message metadata when the bot's user ID appears in the mentions list
• Enables agents to distinguish direct @mentions from background group channel traffic

Changes

• Parse the mentions JSON array from Mattermost WebSocket event data
• Set was_mentioned: true in message metadata when the bot's user ID appears in the mentions list
• Enables agents to distinguish direct @mentions from background group channel traffic

Test plan

• test_parse_mattermost_event_mention_detected — verifies was_mentioned is set when bot ID is in mentions array
• test_parse_mattermost_event_no_mention — verifies metadata is absent when bot is not mentioned
• cargo test --workspace passes (1,744+ tests; one pre-existing flaky cron timing test unrelated to this change)
• cargo clippy --workspace --all-targets -- -D warnings zero warnings
• cargo fmt --all --check no diff

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries — serde_json::from_str(...).unwrap_or_default() handles malformed or missing mentions data by defaulting to an empty vec

[jaberjaber23]

Reviewed and approved. Has merge conflicts with recent changes on main. Please rebase and we will merge.