Add mcp as an AI governance session source

What does this MR do and why?

Add mcp as an AI governance session source

Tool calls through the GitLab MCP server need somewhere to record an inferred session. The source enum names every writer of ai_governance_sessions, so MCP has to appear there before anything can write one.

The GraphQL enum builds its values from Session.sources and fetches a description per key, so a new source without a description raises KeyError at boot. Both move together for that reason.

Changelog: added EE: true

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.