fix(opencode): log tool execution error stack traces

[jpcarranza94]

Issue for this PR

Closes #16925

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When a tool throws during execution, the tool-error handler in processor.ts converts the error to a string via .toString(), which discards the stack trace. No log.error() is emitted either, so the error is invisible in server logs — even at --log-level DEBUG.

This adds a single log.error() call before the existing .toString() conversion. The stack trace appears in the logs, existing behavior is unchanged.

How did you verify your code works?

Ran opencode serve --log-level DEBUG under sustained load (~800 sessions/hour). Before this change, tool errors like RangeError: Maximum call stack size exceeded only appeared embedded inside service=llm log entries. After this change, they show as standalone ERROR entries with full stack traces.

Screenshots / recordings

N/A — server-side logging change, no UI impact.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[jpcarranza94]

The e2e (linux) failure is unrelated to this change — it's a flaky Playwright test in settings-keybinds (terminal toggle keybind works) failing on expect(locator).not.toBeVisible(). This PR only adds a log.error() call in processor.ts and doesn't touch any UI code.