Add JSON-RPC API

What does this MR do and why?

Introduce JSON-RPC API implementation for Model Context Protocol (MCP) server functionality. The implementation follows JSON-RPC specification 2.0 and MCP specification (2025-06-18).

Key additions:

• Feature flag: Add mcp_server to control access to new API (WIP, disabled by default)
• API endpoint: Add POST /api/v4/mcp for JSON-RPC method invocation
• JSON-RPC result/error: Add validations and error handling
• JSON-RPC request handlers: Scaffold initial methods:
  • initialize - Server initialization and capability negotiation
  • notifications/initialized - Client initialization confirmation
  • tools/list - List available tools
  • tools/call - Execute tool operations

References

• Issue: #554939 (closed)
• JSON-RPC Specification: https://www.jsonrpc.org/specification
• Model Context Protocol Specification: https://modelcontextprotocol.io/specification/2025-06-18

How to set up and validate locally

1. In rails console enable the feature flag:

Feature.enable(:mcp_server)

2. Perform JSON-RPC method invocation:

POST /api/v4/mcp
Content-Type: application/json

{"jsonrpc":"2.0","method":"initialize","id":"1"}

3. Confirm JSON-RPC result:

{
  "jsonrpc": "2.0",
  "result": {
    "protocolVersion": "2025-06-18",
    "capabilities": {
      "tools": {
        "listChanged": false
      }
    },
    "serverInfo": {
      "name": "Official GitLab MCP Server",
      "version": "0.0.1-beta"
    }
  },
  "id": "1"
}

Alternatively, you can validate locally end-to-end using client:

Click to expand

1. Configure mcp.json:

{
    "mcpServers": {
      "GitLab": {
        "command": "npx",
        "args": [
          "mcp-remote",
          "http://127.0.0.1:3000/api/v4/mcp",
          "--allow-http",
          "--debug",
          "--header",
          "Authorization:${AUTH_HEADER}"
        ],
        "env": {
          "AUTH_HEADER": "Temporary workaround. See: https://gitlab.com/gitlab-org/gitlab/-/issues/554826"
        }
      }
    }
  }

2. Manually list and call tools from MCP client:

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