test: add realtime tool output serialization edge cases

Add focused realtime serialization coverage for _serialize_tool_output.

- serialize None as JSON null
- serialize lists as JSON arrays
- serialize dataclass instances via asdict
- fall back to string output for raw bytes

Author: Aphroq

tests/realtime/test_session.py

@@ -1505,6 +1505,27 @@ def __str__(self) -> str:
 
         assert _serialize_tool_output(BrokenDataclass(lock=threading.Lock())) == "broken-dataclass"
 
+    def test_serialize_tool_output_serializes_none_as_json_null(self) -> None:
+        assert _serialize_tool_output(None) == "null"
+
+    def test_serialize_tool_output_serializes_lists_as_json(self) -> None:
+        value = ["hello", 1, True, None]
+
+        assert _serialize_tool_output(value) == json.dumps(value)
+
+    def test_serialize_tool_output_serializes_dataclass_instances(self) -> None:
+        @dataclasses.dataclass
+        class ToolResult:
+            label: str
+            values: list[int]
+
+        assert _serialize_tool_output(ToolResult(label="demo", values=[1, 2])) == json.dumps(
+            {"label": "demo", "values": [1, 2]}
+        )
+
+    def test_serialize_tool_output_returns_string_for_bytes(self) -> None:
+        assert _serialize_tool_output(b"abc") == "b'abc'"
+
     @pytest.mark.asyncio
     async def test_mixed_tool_types_filtering(self, mock_model, mock_agent):
         """Test that function tools and handoffs are properly separated"""