Describe the bug

Summary
When a tool‑calling model returns an invalid tool call, the framework raises a FormatError. The exception payload can include non‑JSON‑serializable
objects (e.g., raw response objects), which causes agent.save() to fail. As a result, the final .traj.json is never written, even though
trajectories are normally always persisted.

Observed behavior

• Invalid tool call → FormatError
• agent.save() attempts to json.dumps() the trajectory
• Serialization fails due to non‑JSON‑serializable data in the error payload
• Trajectory file is not written to disk

Expected behavior

• Trajectories should always be written, even when the model response is malformed
• Error payloads should be JSON‑safe

Impact
Losing the trajectory on malformed tool calls makes debugging and evaluation difficult, and breaks expected SWE‑bench behavior.

Fix (coming soon in a PR)

• Wrap FormatError with a JSON‑safe payload (raw assistant content/tool_calls only + minimal metadata)
• Add a to_jsonable helper and use it before json.dumps() in DefaultAgent.save() so serialization is robust