Return granular per-field diffs for MCP tool updates

- Motivation:
When tools change, users should see exactly what changed (e.g., description, parameters) rather than just a tool name.

- Proposed solution:
Extend `MCPToolsDiff` (in `backend/aci/control_plane/services/mcp_tools/mcp_tools_manager.py`) to include per-field differences; surface via `POST /{id}/refresh-tools` in `backend/aci/control_plane/routes/mcp_servers.py`; render diffs in the detail view UI.

- Additional context:
- Consider limiting diff size and sensitive fields.
- Frontend: `frontend/src/app/(dashboard)/mcp-servers/[id]/page.tsx`.