Python: Fix response_format crash on background polling with empty text (#5146)

* Guard against empty text in _parse_structured_response_value (#5145)

When using response_format with background=True (Responses API), polling
an in-progress response produces empty text. _parse_structured_response_value
unconditionally passed this to model_validate_json/json.loads, causing
ValidationError or JSONDecodeError.

Add an early return of None when text is empty, matching the existing
guard for response_format=None. This allows .value to safely return None
for in-progress background responses.

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

* Python: Fix `response_format` crash on background polling with empty text

Fixes #5145

---------

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

python/packages/core/tests/core/test_types.py

@@ -39,6 +39,7 @@ from agent_framework._types import (
     _get_data_bytes,
     _get_data_bytes_as_str,
     _parse_content_list,
+    _parse_structured_response_value,
     _validate_uri,
     add_usage_details,
     validate_tool_mode,
@@ -813,6 +814,32 @@ def test_chat_response_with_mapping_response_format() -> None:
     assert response.value["response"] == "Hello"
 
 
+def test_parse_structured_response_value_empty_text_with_pydantic_model() -> None:
+    """Empty text should return None instead of raising when response_format is a Pydantic model."""
+    result = _parse_structured_response_value("", OutputModel)
+    assert result is None
+
+
+def test_parse_structured_response_value_empty_text_with_mapping() -> None:
+    """Empty text should return None instead of raising when response_format is a mapping."""
+    result = _parse_structured_response_value("", {"type": "object"})
+    assert result is None
+
+
+def test_chat_response_value_with_empty_text_and_response_format() -> None:
+    """ChatResponse.value should return None when text is empty and response_format is set."""
+    message = Message(role="assistant", contents=[""])
+    response = ChatResponse(messages=message, response_format=OutputModel)
+    assert response.value is None
+
+
+def test_agent_response_value_with_empty_text_and_response_format() -> None:
+    """AgentResponse.value should return None when text is empty and response_format is set."""
+    message = Message(role="assistant", contents=[""])
+    response = AgentResponse(messages=message, response_format=OutputModel)
+    assert response.value is None
+
+
 def test_chat_response_value_raises_on_invalid_schema():
     """Test that value property raises ValidationError with field constraint details."""