fix(core): reuse connected MCP client in subagent invocations

Subsequent subagent invocations attempt to connect to an already connected MCP client, throwing an error. This commit adds a status check before calling client.connect(), enabling client.discoverInto() to successfully populate the new subagent registry.

packages/core/src/tools/mcp-client-manager.test.ts

@@ -14,7 +14,7 @@ import {
   type MockedObject,
 } from 'vitest';
 import { McpClientManager } from './mcp-client-manager.js';
-import { McpClient, MCPDiscoveryState } from './mcp-client.js';
+import { McpClient, MCPDiscoveryState, MCPServerStatus } from './mcp-client.js';
 import type { ToolRegistry } from './tool-registry.js';
 import type { Config, GeminiCLIExtension } from '../config/config.js';
 import type { PromptRegistry } from '../prompts/prompt-registry.js';
@@ -38,7 +38,7 @@ describe('McpClientManager', () => {
       connect: vi.fn(),
       discoverInto: vi.fn(),
       disconnect: vi.fn(),
-      getStatus: vi.fn(),
+      getStatus: vi.fn().mockReturnValue(MCPServerStatus.DISCONNECTED),
       getServerConfig: vi.fn(),
       getServerName: vi.fn().mockReturnValue('test-server'),
     } as unknown as McpClient);

packages/core/src/tools/mcp-client-manager.ts

@@ -13,6 +13,7 @@ import type { ToolRegistry } from './tool-registry.js';
 import {
   McpClient,
   MCPDiscoveryState,
+  MCPServerStatus,
   populateMcpServerCommand,
 } from './mcp-client.js';
 import { getErrorMessage, isAuthenticationError } from '../utils/errors.js';
@@ -458,7 +459,9 @@ export class McpClientManager {
               : undefined);
 
           try {
-            await client.connect();
+            if (client.getStatus() === MCPServerStatus.DISCONNECTED) {
+              await client.connect();
+            }
             if (targetRegistries) {
               await client.discoverInto(this.cliConfig, targetRegistries);
             }