test: tag @metamask-qa-team in feature flag drift Slack noti

[LeVinhGithub]

Summary

Updates the reusable feature-flag-drift-slack-noti action so drift Slack messages mention @metamask-qa-team, pinging the whole QA user group instead of posting a silent channel message that is easy to miss.

Changes

• slack-subteam: New optional input (default S0832MYT8QZ / @metamask-qa-team). Pass empty to skip the mention.
• Slack payload uses <!subteam^ID> so incoming webhooks actually notify the user group.
• Mention is appended as the last Block Kit section (not in the title), and also included in the top-level text fallback so Slack push notifications still fire.
• Sets link_names: true as extra insurance for mention parsing.

Testing

• Trigger a feature-flag drift Slack notification and confirm @metamask-qa-team appears at the end of the message (not in the title).
• Confirm members of @metamask-qa-team receive a Slack ping/notification.
• Pass slack-subteam: '' and confirm the mention is omitted.

Fixes

Fixes: N/A

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Note

Low Risk
Changes only affect GitHub Action Slack notification formatting and optional QA group mentions, with no impact on application runtime or security-sensitive paths.

Overview
Feature flag drift Slack notifications now ping @metamask-qa-team by default so drift alerts are harder to miss in channel-only posts.

The feature-flag-drift-slack-noti composite action adds an optional slack-subteam input (default user group S0832MYT8QZ; pass empty to skip). The payload builder constructs a <!subteam^ID> mention for incoming webhooks, keeps the headline unchanged, appends the mention as the last Block Kit section, duplicates it in top-level text for push notifications, and sets link_names: true. Both PR and non-PR payload paths use the same mention logic.

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