Add AI Governance UI for external MCP server blocks

What does this MR do and why?

Add AI Governance UI for external MCP server blocks

Add an MCP registry tab to the group and project AI Governance area: a table listing external MCP servers with their block status and a Block/Allow toggle, backed by the aiCatalogMcpServers listing and the AiCatalogMcpServerSetBlock mutation. Inherited ancestor-group blocks are shown read-only.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/603573

Changelog: added EE: true

References

Screenshots or screen recordings

Demo video — the new MCP registry tab on the group AI Governance page: blocking an external MCP server for the whole group, allowing it again, then the project-level Governance page where the group block is shown as read-only (blocked by ancestor):

How to set up and validate locally

MR acceptance checklist

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