refactor: Extract MCP discovery from ToolRegistry

- Moves MCP tool discovery logic from ToolRegistry into a new, dedicated MCP client (mcp-client.ts and mcp-tool.ts).
- Updates ToolRegistry to utilize the new MCP client.
- Adds comprehensive tests for the new MCP client and its integration with ToolRegistry.

Part of https://github.com/google-gemini/gemini-cli/issues/577

packages/server/src/tools/mcp-tool.ts

@@ -0,0 +1,49 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { BaseTool, ToolResult } from './tools.js';
+
+type ToolParams = Record<string, unknown>;
+
+export const MCP_TOOL_DEFAULT_TIMEOUT_MSEC = 10 * 60 * 1000; // default to 10 minutes
+
+export class DiscoveredMCPTool extends BaseTool<ToolParams, ToolResult> {
+  constructor(
+    private readonly mcpClient: Client,
+    readonly name: string,
+    readonly description: string,
+    readonly parameterSchema: Record<string, unknown>,
+    readonly serverToolName: string,
+    readonly timeout?: number,
+  ) {
+    description += `
+
+This MCP tool was discovered from a local MCP server using JSON RPC 2.0 over stdio transport protocol.
+When called, this tool will invoke the \`tools/call\` method for tool name \`${name}\`.
+MCP servers can be configured in project or user settings.
+Returns the MCP server response as a json string.
+`;
+    super(name, name, description, parameterSchema);
+  }
+
+  async execute(params: ToolParams): Promise<ToolResult> {
+    const result = await this.mcpClient.callTool(
+      {
+        name: this.serverToolName,
+        arguments: params,
+      },
+      undefined, // skip resultSchema to specify options (RequestOptions)
+      {
+        timeout: this.timeout ?? MCP_TOOL_DEFAULT_TIMEOUT_MSEC,
+      },
+    );
+    return {
+      llmContent: JSON.stringify(result, null, 2),
+      returnDisplay: JSON.stringify(result, null, 2),
+    };
+  }
+}