Python: preserve A2A message context_id (#4686)

* Python: forward A2A context_id

* Avoid duplicating A2A context ids

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

---------

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

python/packages/a2a/tests/test_a2a_agent.py

@@ -507,6 +507,23 @@ def test_prepare_message_for_a2a_with_multiple_contents() -> None:
     assert result.parts[3].root.kind == "text"  # JSON text remains as text (no parsing)
 
 
+def test_prepare_message_for_a2a_forwards_context_id() -> None:
+    """Test conversion of Message preserves context_id without duplicating it in metadata."""
+
+    agent = A2AAgent(client=MagicMock(), _http_client=None)
+
+    message = Message(
+        role="user",
+        contents=[Content.from_text(text="Continue the task")],
+        additional_properties={"context_id": "ctx-123", "trace_id": "trace-456"},
+    )
+
+    result = agent._prepare_message_for_a2a(message)
+
+    assert result.context_id == "ctx-123"
+    assert result.metadata == {"trace_id": "trace-456"}
+
+
 def test_parse_contents_from_a2a_with_data_part() -> None:
     """Test conversion of A2A DataPart."""