feat(chat): MCP stdio 工具接入

- McpServer：启动进程、initialize、tools/list、tools/call
- McpManager：多服务器、工具名冲突处理、OpenAI function 定义
- ChatClient：请求携带 tools，流式累积 tool_calls，结束后交回上层
- ChatWindow：工具调用循环（最多 5 轮）、scaffold 消息、设置里的 MCP JSON 编辑
- 无头自测 gxde-ai-see-mcptest + tests/mcp/test_server.py 验证通过

tests/mcp/test_server.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2026 GXDE OS Team
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""Tiny stdio MCP server used by tests/mcptest.cpp."""
+
+import json
+import sys
+
+
+def reply(message, result):
+    print(json.dumps({"jsonrpc": "2.0", "id": message["id"], "result": result}), flush=True)
+
+
+for line in sys.stdin:
+    line = line.strip()
+    if not line:
+        continue
+    message = json.loads(line)
+    method = message.get("method")
+
+    if method == "initialize":
+        reply(message, {
+            "protocolVersion": "2024-11-05",
+            "capabilities": {"tools": {}},
+            "serverInfo": {"name": "test-server", "version": "0.1"},
+        })
+    elif method == "tools/list":
+        reply(message, {
+            "tools": [{
+                "name": "echo",
+                "description": "Echo the given text back",
+                "inputSchema": {
+                    "type": "object",
+                    "properties": {"text": {"type": "string"}},
+                    "required": ["text"],
+                },
+            }]
+        })
+    elif method == "tools/call":
+        arguments = message.get("params", {}).get("arguments", {})
+        reply(message, {
+            "content": [{"type": "text", "text": "echo: " + str(arguments.get("text", ""))}]
+        })
+    elif "id" in message:
+        reply(message, {})