prompts: avoid wrapping file attachments in code fences (#569)

* 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