fix(streaming): apply structured logging format and add warning test

- Switch warning to project's field=<value> | message convention per AGENTS.md
- Add dedicated test that asserts logger.warning is emitted with the
  expected tool_name and raw_input on malformed JSON

Address auto-review feedback on #2054.

Author: Ratansairohith

src/strands/event_loop/streaming.py

@@ -280,7 +280,7 @@ def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
             current_tool_use["input"] = json.loads(current_tool_use["input"])
         except ValueError:
             logger.warning(
-                "Failed to parse tool input JSON for '%s': %s",
+                "tool_name=<%s>, raw_input=<%s> | failed to parse tool input json, defaulting to empty dict",
                 current_tool_use.get("name", "unknown"),
                 current_tool_use["input"][:200] if isinstance(current_tool_use.get("input"), str) else "",
             )

tests/strands/event_loop/test_streaming.py

@@ -546,6 +546,25 @@ def test_handle_content_block_stop(state, exp_updated_state):
     assert tru_updated_state == exp_updated_state
 
 
+@unittest.mock.patch("strands.event_loop.streaming.logger")
+def test_handle_content_block_stop_logs_warning_on_malformed_json(mock_logger):
+    state = {
+        "content": [],
+        "current_tool_use": {"toolUseId": "123", "name": "test_tool", "input": "{invalid json}"},
+        "text": "",
+        "reasoningText": "",
+        "citationsContent": [],
+        "redactedContent": b"",
+    }
+
+    strands.event_loop.streaming.handle_content_block_stop(state)
+
+    mock_logger.warning.assert_called_once()
+    call_args = mock_logger.warning.call_args
+    assert "test_tool" in str(call_args)
+    assert "{invalid json}" in str(call_args)
+
+
 def test_handle_message_stop():
     event: MessageStopEvent = {"stopReason": "end_turn"}