Support for MCP env config

[leaft]

Validations

• I believe this is a way to improve. I'll try to join the Continue Discord for questions
• I'm not able to find an open issue that requests the same enhancement

Problem

I have checked the code on main branch, found cannot set env in transport config now,

continue/core/context/mcp/index.ts

Lines 71 to 75 in 5251852

	case "stdio":
	return new StdioClientTransport({
	command: options.transport.command,
	args: options.transport.args,
	});

Solution

Support env block in config.json, add PATH environment and env in config when construct the StdioClientTransport.
Example config.json and code below.

{
// ... other config
  "experimental": {
    "useTools": true,
    "modelContextProtocolServers": [
      {
        "transport": {
          "type": "stdio",
          "command": "npx",
          "args": [
            "-y",
            "@modelcontextprotocol/server-brave-search"
          ],
          "env": {
            "BRAVE_API_KEY": "xxx...xxx"
          }
        }
      }
    ]
  }
}

new StdioClientTransport({ 
    command: options.transport.command, 
    args: options.transport.args, 
    env: {
	...options.transport.env,
	...(process.env.PATH ? { PATH: process.env.PATH } : {}),
      },
   }); 

[sestinj]

@leaft thanks for calling this out! I just made the fix here: #3600

and am about to push a new pre-release version for VS Code in the next hour