Add support for MCP server instructions behind config option (#13432)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/core/src/config/config.ts

@@ -203,6 +203,8 @@ export class MCPServerConfig {
     readonly targetAudience?: string,
     /* targetServiceAccount format: <service-account-name>@<project-num>.iam.gserviceaccount.com */
     readonly targetServiceAccount?: string,
+    // Include the MCP server initialization instructions in the system instructions
+    readonly useInstructions?: boolean,
   ) {}
 }