Slack MCP Connector: Add support for mark-as-read (conversations.mark)

Feature Request

Currently the Slack MCP connector does not expose conversations.mark, which would allow marking messages or channels as read.

Use case

A Slack triage agent reads all unread messages, categorizes them into 'important' and 'nice to know', summarizes both in chat, and then automatically marks 'nice to know' messages as read — reducing notification noise for the user.

Requested addition

• slack_mark_as_read wrapping Slack's conversations.mark API endpoint — mark a channel (or up to a specific message timestamp) as read

Why this matters

Without this, triage automation can summarize messages but cannot close the loop by clearing low-priority notifications. The mark-as-read action is what makes the workflow feel complete rather than just informational.