#10081: fix(mattermost): pass requireMentionOverride to core resolver

## Summary When `account.requireMention` is set to `false` in config, the Mattermost monitor should pass this override to `resolveRequireMention()` so that channels configured with `requireMention: false` respond without needing an @mention. ## Problem Without this fix, the account-level `requireMention` setting is ignored and the default (`true`) is used, requiring @mentions even when configured otherwise. ## Solution Pass `requireMentionOverride: account.requireMention` to the `core.channel.groups.resolveRequireMention()` call. ## Testing 1. Configure Mattermost account with `requireMention: false` 2. Send a message in a channel without @mentioning the bot 3. Bot should respond (before this fix, it would not) --- *Resubmission of #5068 rebased on latest main* <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Mattermost monitor to pass the account-level `requireMention` setting through to `core.channel.groups.resolveRequireMention()`. This ensures group channels can respond without an @mention when `channels.mattermost.accounts.<id>.requireMention: false` is configured, aligning Mattermost with other monitors that already forward a mention override into the shared group-policy resolver. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is a single-parameter pass-through in the Mattermost monitor, and the target resolver (`resolveChannelGroupRequireMention`) explicitly supports `requireMentionOverride` as an optional boolean. No other behavior is modified. - No files require special attention <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->