New untitled files are not correctly managed by config file status bar indicator

@DanielRosenwasser がすでに取り組んでいます。

2026年4月28日 から。

1. Have an open `.ts` file with an associated project. The indicator should show `[.../]tsconfig.json`.
2. Create a new unsaved/untitled buffer in VS Code
3. Add some TS code into this file, or "Change Language Mode" to "TypeScript".
4. Notice that even though this is an unsaved file, the status bar will still show the old `tsconfig.json` file in the project indicator.
5. If you switch focus to another editor and switch back, you'll be left with "No tsconfig"