Fix sandbox execPath resolution for remote environments

Add execPath to IRemoteAgentEnvironment so the server sends its actual
process.execPath to the client. The sandbox service now uses this instead
of hardcoding appRoot + '/node', which only works in production builds.

src/vs/workbench/contrib/debug/test/browser/debugConfigurationManager.test.ts

@@ -137,6 +137,7 @@ suite('debugConfigurationManager', () => {
 					pid: 1,
 					connectionToken: 'token',
 					appRoot: URI.file('/remote/app'),
+					execPath: '/remote/app/node',
 					tmpDir: URI.file('/remote/tmp'),
 					settingsPath: URI.file('/remote/settings.json'),
 					mcpResource: URI.file('/remote/mcp.json'),