This format is enabled for Google models as they seem to prefer it.
A relevant unit eval's pass rate has increased from 0.77 to 0.98.

Diff-fenced format looks like this (markdown fences and a line hint are
optional):

```diff
<<<<<<< SEARCH line=42
...
=======
...
>>>>>>> REPLACE
```

Release Notes:

- Agent: Gemini models now use the diff-fenced format when making edits