fix: recover malformed JSON arguments in write_file tool calls

[jigangz]

Summary

Fix write_file tool reporting 'missing path' even when the path parameter is explicitly provided by the LLM.

Problem

When LLMs produce slightly malformed JSON arguments (e.g. trailing commas), json.loads() fails silently and falls back to args = {}. The empty-args guard then returns an error to the LLM saying the path is missing, even though the LLM did provide it.

Changes

• Better logging: Log the raw argument string and parse error details when JSON parsing fails
• Fallback cleanup: Attempt to fix common LLM JSON errors (trailing commas before } or ]) before giving up
• No behavior change for valid JSON — only affects recovery from malformed input

Testing

• Valid JSON arguments: no change in behavior
• Malformed JSON with trailing commas: now recovers instead of silently dropping args
• Completely broken JSON: still falls back to empty args with better logging

Fixes #177