feat(test-utils): add TestMcpServerBuilder and support in TestRig (#23491)

packages/test-utils/GEMINI.md

@@ -10,6 +10,58 @@ published to npm.
 - `src/file-system-test-helpers.ts`: Helpers for creating temporary file system
   fixtures.
 - `src/mock-utils.ts`: Common mock utilities.
+- `src/test-mcp-server.ts`: Helper for building test MCP servers for tests.
+- `src/test-mcp-server-template.mjs`: Generic template script for running
+  isolated MCP processes.
+
+## Test MCP Servers
+
+The `TestRig` provides a fully isolated, compliant way to test tool triggers and
+workflows using local test MCP servers. This isolates your tests from live API
+endpoints and rate-limiting.
+
+### Usage
+
+1. **Programmatic Builder:**
+
+   ```typescript
+   import { TestMcpServerBuilder } from '@google/gemini-cli-test-utils';
+
+   const builder = new TestMcpServerBuilder('weather-server').addTool(
+     'get_weather',
+     'Get weather',
+     'It is rainy',
+   );
+
+   rig.addTestMcpServer('weather-server', builder.build());
+   ```
+
+2. **Predefined configurations via JSON:** Place a configuration file in
+   `packages/test-utils/assets/test-servers/google-workspace.json` and load it
+   by title:
+
+   ```typescript
+   rig.addTestMcpServer('workspace-server', 'google-workspace');
+   ```
+
+   **JSON Format Structure (`TestMcpConfig`):**
+
+   ```json
+   {
+     "name": "string (Fallback server name)",
+     "tools": [
+       {
+         "name": "string (Tool execution name)",
+         "description": "string (Helpful summary for router)",
+         "inputSchema": {
+           "type": "object",
+           "properties": { ... }
+         },
+         "response": "string | object (The forced reply payload)"
+       }
+     ]
+   }
+   ```
 
 ## Usage