* prompts: avoid wrapping file attachments in code fences
We previously included attached files like this
```
<attachment id="test.js" filePath="/home/jola/test.js">
\```javascript
bar
asd (
\```
</attachment>
```
Which, very reasonably, caused the model to get confused and think that
the file actually contained those backticks. This change removes that
so that it's just
```
<attachment id="test.js" filePath="/home/jola/test.js">
bar
asd (
</attachment>
```
Refs https://github.com/microsoft/vscode/issues/260772#issuecomment-3176793591
Busted all the caches with associated baseline noise.
* fixup
Rob Lourens
Connor Peet
kieferrmandcopilot-swe-agent[bot]