Python: Fix MCP tool result serialization for list[TextContent] (#2523)

* Fix MCP tool result serialization for list[TextContent]

When MCP tools return results containing list[TextContent], they were
incorrectly serialized to object repr strings like:
'[<agent_framework._types.TextContent object at 0x...>]'

This fix properly extracts text content from list items by:
1. Checking if items have a 'text' attribute (TextContent)
2. Using model_dump() for items that support it
3. Falling back to str() for other types
4. Joining single items as plain text, multiple items as JSON array

Fixes #2509

* Address PR review feedback for MCP tool result serialization

- Extract serialize_content_result() to shared _utils.py
- Fix logic: use texts[0] instead of join for single item
- Add type annotation: texts: list[str] = []
- Return empty string for empty list instead of '[]'
- Move import json to file top level
- Add comprehensive unit tests for serialization

* Address PR review feedback: fix type checking and double serialization

- Add isinstance(item.text, str) check to ensure text attribute is a string
- Fix double-serialization issue by keeping model_dump results as dicts
  until final json.dumps (removes escaped JSON strings in arrays)
- Improve docstring with detailed return value documentation
- Add test for non-string text attribute handling
- Add tests for list type tool results in _events.py path

* Simplify PR: minimal changes to fix MCP tool result serialization

Addresses reviewer feedback about excessive refactoring:
- Reset _events.py to original structure
- Only add import and use serialize_content_result in one location
- All review comments addressed in serialize_content_result():
  - Added isinstance(item.text, str) check
  - Use model_dump(mode="json") to avoid double-serialization
  - Improved docstring with explicit return value documentation
  - Empty list returns "" instead of "[]"

* Refactor: Move MCP TextContent serialization to core prepare_function_call_results

Per reviewer feedback, moved the TextContent serialization logic from
ag-ui's serialize_content_result to the core package's
prepare_function_call_results function.

Changes:
- Added handling for objects with 'text' attribute (like MCP TextContent)
  in _prepare_function_call_results_as_dumpable
- Removed serialize_content_result from ag-ui/_utils.py
- Updated _events.py and _message_adapters.py to use
  prepare_function_call_results from core package
- Updated tests to match the core function's behavior

* Fix failing tests for prepare_function_call_results behavior

- test_tool_result_with_none: Update expected value to 'null' (JSON serialization of None)
- test_tool_result_with_model_dump_objects: Use Pydantic BaseModel instead of plain class

* Fix B903 linter error: Convert MockTextContent to dataclass

The ruff linter was reporting B903 (class could be dataclass or namedtuple)
for the MockTextContent test helper classes. This commit converts them to
dataclasses to satisfy the linter check.

python/packages/ag-ui/tests/test_message_adapters.py

@@ -3,7 +3,7 @@
 """Tests for message adapters."""
 
 import pytest
-from agent_framework import ChatMessage, FunctionCallContent, Role, TextContent
+from agent_framework import ChatMessage, FunctionCallContent, FunctionResultContent, Role, TextContent
 
 from agent_framework_ag_ui._message_adapters import (
     agent_framework_messages_to_agui,
@@ -278,3 +278,119 @@ def test_extract_text_from_custom_contents():
     result = extract_text_from_contents(contents)
 
     assert result == "Custom Mixed"
+
+
+# Tests for FunctionResultContent serialization in agent_framework_messages_to_agui
+
+
+def test_agent_framework_to_agui_function_result_dict():
+    """Test converting FunctionResultContent with dict result to AG-UI."""
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[FunctionResultContent(call_id="call-123", result={"key": "value", "count": 42})],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    assert agui_msg["role"] == "tool"
+    assert agui_msg["toolCallId"] == "call-123"
+    assert agui_msg["content"] == '{"key": "value", "count": 42}'
+
+
+def test_agent_framework_to_agui_function_result_none():
+    """Test converting FunctionResultContent with None result to AG-UI."""
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[FunctionResultContent(call_id="call-123", result=None)],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    # None serializes as JSON null
+    assert agui_msg["content"] == "null"
+
+
+def test_agent_framework_to_agui_function_result_string():
+    """Test converting FunctionResultContent with string result to AG-UI."""
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[FunctionResultContent(call_id="call-123", result="plain text result")],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    assert agui_msg["content"] == "plain text result"
+
+
+def test_agent_framework_to_agui_function_result_empty_list():
+    """Test converting FunctionResultContent with empty list result to AG-UI."""
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[FunctionResultContent(call_id="call-123", result=[])],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    # Empty list serializes as JSON empty array
+    assert agui_msg["content"] == "[]"
+
+
+def test_agent_framework_to_agui_function_result_single_text_content():
+    """Test converting FunctionResultContent with single TextContent-like item."""
+    from dataclasses import dataclass
+
+    @dataclass
+    class MockTextContent:
+        text: str
+
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[FunctionResultContent(call_id="call-123", result=[MockTextContent("Hello from MCP!")])],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    # TextContent text is extracted and serialized as JSON array
+    assert agui_msg["content"] == '["Hello from MCP!"]'
+
+
+def test_agent_framework_to_agui_function_result_multiple_text_contents():
+    """Test converting FunctionResultContent with multiple TextContent-like items."""
+    from dataclasses import dataclass
+
+    @dataclass
+    class MockTextContent:
+        text: str
+
+    msg = ChatMessage(
+        role=Role.TOOL,
+        contents=[
+            FunctionResultContent(
+                call_id="call-123",
+                result=[MockTextContent("First result"), MockTextContent("Second result")],
+            )
+        ],
+        message_id="msg-789",
+    )
+
+    messages = agent_framework_messages_to_agui([msg])
+
+    assert len(messages) == 1
+    agui_msg = messages[0]
+    # Multiple items should return JSON array
+    assert agui_msg["content"] == '["First result", "Second result"]'