Draft: Add MCP tool support to frontend agent form

What does this MR do and why?

This is the frontend counterpart to !224998 (merged) (backend). Per reviewer feedback about multiversion compatibility, the backend and frontend changes were split into separate MRs.

This MR adds MCP tool selection support to the AI Catalog agent form frontend.

Original issue: #584281

References

Screenshots or screen recordings

Enabling GitLab MCP Server tools
Turning off GitLab Duo Experimental Setting

How to set up and validate locally

1. Ensure you have HTTPs enabled within you GDK to run any MCP Server

https:
  enabled: true
nginx:
  enabled: true
  http:
    enabled: true
    port: 8080
  http2:
    enabled: true
  ssl:
    certificate: gdk.test.pem
    key: gdk.test-key.pem

Run gdk reconfigure and gdk restart to enable the following https usage in the GDK.

1. Enable both required feature flags in your GDK Rails console:

   Feature.enable(:mcp_client)
   Feature.enable(:mcp_catalog_agent_tools)

2. Ensure the Duo experimental features admin setting is enabled at /admin/application_settings/general.

3. Test the agent catalog — see AI catalog MCP servers:

   • Navigate to a project's Automate > Agents page and create or edit an agent.
   • Open the tool selection dropdown — GitLab MCP Server tools should appear alongside DAP built-in tools.
   • Select one or more MCP tools and save the agent.
   • Confirm a workflow run for that agent only uses the selected MCP tools.