Track file type or extension for failing requests

@DanielRosenwasser がすでに取り組んでいます。

2026年3月25日 から。

• #3228 @copilot-swe-agent による — マージされずにクローズ

Often it is a little bit easier to reproduce a request failure if we know the file type that triggered a crash (e.g. if something happened in a `.js` file, it's likely that some special-cased JS-pattern is at fault). We should try to surface this information when possible (e.g. on requests that provide a `textDocument` property).