Compare Expected vs Actual Tool Traces

Implement compare_traces(expected, actual) that compares two lists of trace steps. Each step is a dict: {"step": int, "tool": str, "args": str}. Compare step-by-step in order. Steps with the same index must match exactly. If the lists differ in length, extra or missing steps are mismatches. Return {"match": bool, "mismatches": [{"step": int, "expected": dict|null, "actual": dict|null}]}. Each mismatch entry identifies the step index and shows what was expected vs what was actually produced.