fix: use AsyncMock to fix coverage tracking in CI

Replace inner async function with patch.object + AsyncMock to avoid
coverage blind spot when the mock runs inside the server's task
context with pytest-xdist parallel workers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Varun Sharma

tests/server/mcpserver/test_server.py

@@ -307,20 +307,16 @@ async def test_handle_call_tool_defensive_exception_handler(self):
         mcp = MCPServer()
         mcp.add_tool(tool_fn)
 
-        original_call_tool = mcp.call_tool
-
-        async def patched_call_tool(name: str, arguments: dict[str, Any]) -> Any:
-            raise RuntimeError("internal db connection string leaked")
-
         async with Client(mcp) as client:
-            mcp.call_tool = patched_call_tool  # type: ignore[assignment]
-            result = await client.call_tool("tool_fn", {"x": 1, "y": 2})
-            mcp.call_tool = original_call_tool  # type: ignore[assignment]
-            assert result.is_error is True
-            content = result.content[0]
-            assert isinstance(content, TextContent)
-            assert "unexpected error" in content.text.lower()
-            assert "internal db connection string leaked" not in content.text
+            with patch.object(
+                mcp, "call_tool", new_callable=AsyncMock, side_effect=RuntimeError("internal db leak")
+            ):
+                result = await client.call_tool("tool_fn", {"x": 1, "y": 2})
+                assert result.is_error is True
+                content = result.content[0]
+                assert isinstance(content, TextContent)
+                assert "unexpected error" in content.text.lower()
+                assert "internal db leak" not in content.text
 
     async def test_tool_return_value_conversion(self):
         mcp = MCPServer()