fix unterminated string errors in workflow edit tool args

[sudorest]

Summary

GitLab Duo workflow edit-writing flows can fail with unterminated string errors when tool arguments are wrapped in fenced JSON or surrounding prose instead of being emitted as a strict JSON object.

Reproduction

1. Use a GitLab workflow-backed model in opencode.
2. Trigger an edit-writing flow that emits a tool call with multiline arguments.
3. Observe the tool execution path fail before the tool runs when the arguments are wrapped or malformed.

Expected

The workflow tool executor should tolerate common wrapper formats around object-shaped JSON tool args, while still rejecting malformed inputs cleanly.

Actual

The executor calls JSON.parse directly on the raw args string, which surfaces parser failures like unterminated string during edit-writing flows.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Tool call arguments may contain invalid JSON #17750: Tool call arguments may contain invalid JSON (same root cause — tool args not validated before JSON.parse, affecting streaming and non-streaming modes)
• Bug: Incomplete JSON stream in OpenCode using gpt-5.3-codex (Unterminated string) #20599: Bug: Incomplete JSON stream / Unterminated string error in tool calls
• [BUG] openai/gpt-5.2-codex Error message: JSON Parse error: Unterminated string errors #10755: JSON Parse error: Unterminated string errors during tool calls

If your issue is distinct (specifically about GitLab Duo workflow wrapping args in fenced JSON/prose), please clarify the difference in a comment.