[CLI] Expand variables in MCP configuration

[langri-sha]

I'd like to request if support could be added for expanding variables in the mcp.json configuration, e.g:

   "context7": {
      "type": "http",
      "url": "https://mcp.context7.com/mcp",
      "headers": {
        "CONTEXT7_API_KEY": "${CONTEXT7_API_KEY}"
      },
      "disabled": false
    },

Why is this important? It's great for users that are configuring secrets from a secret store.

Certain agents already support this.

[factory-davidgu]

This is now working!