fix(mcp): combine content and structuredContent when both present (#7118)

When an MCP server returns both content (model-oriented text) and
structuredContent (machine-oriented JSON), the client now combines
them instead of discarding content.  The text content becomes the
primary result (what the agent reads), and structuredContent is
included as supplementary metadata.

Previously, structuredContent took full precedence — causing data
loss for servers like Desktop Commander that put the actual file
text in content and metadata in structuredContent.

MCP spec guidance: for conversational/agent UX, prefer content.

tests/tools/test_mcp_structured_content.py

@@ -66,8 +66,8 @@ class TestStructuredContentPreservation:
         data = json.loads(raw)
         assert data == {"result": "hello"}
 
-    def test_structured_content_is_the_result(self, _patch_mcp_server):
-        """When structuredContent is present, it becomes the result directly."""
+    def test_both_content_and_structured(self, _patch_mcp_server):
+        """When both content and structuredContent are present, combine them."""
         session = _patch_mcp_server
         payload = {"value": "secret-123", "revealed": True}
         session.call_tool = AsyncMock(
@@ -79,7 +79,27 @@ class TestStructuredContentPreservation:
         handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
         raw = handler({})
         data = json.loads(raw)
-        assert data["result"] == payload
+        # content is the primary result, structuredContent is supplementary
+        assert data["result"] == "OK"
+        assert data["structuredContent"] == payload
+
+    def test_both_content_and_structured_desktop_commander(self, _patch_mcp_server):
+        """Real-world case: Desktop Commander returns file text in content,
+        metadata in structuredContent.  Agent must see file contents."""
+        session = _patch_mcp_server
+        file_text = "import os\nprint('hello')\n"
+        metadata = {"fileName": "main.py", "filePath": "/tmp/main.py", "fileType": "python"}
+        session.call_tool = AsyncMock(
+            return_value=_FakeCallToolResult(
+                content=[_FakeContentBlock(file_text)],
+                structuredContent=metadata,
+            )
+        )
+        handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
+        raw = handler({})
+        data = json.loads(raw)
+        assert data["result"] == file_text
+        assert data["structuredContent"] == metadata
 
     def test_structured_content_none_falls_back_to_text(self, _patch_mcp_server):
         """When structuredContent is explicitly None, fall back to text."""