Problems panel shows problems in virtual and read only file

**Describe the bug**

The problems panel is showing problems in a virtual and read only file:

When I copy the problem and paste the contents into a new file, I see this:

```
"resource": "/commit~cf17f9d0/.github/workflows/build.yml",
"owner": "_generated_diagnostic_collection_name_#3",
```

`/commit~cf17f9d0/.github/workflows/build.yml` is not a real file (at least not in my project's directory).

When I click on one of these problems to open the file and then I try to make changes, VS Code reports that it's a read only file:

The problems disappear when I uninstall or disable the extension.

The problems re-appear when I re-install or re-enable the extension.

**To Reproduce**
Unfortunately I don't have steps to reproduce this. I'm not sure how it happened.

**Expected behavior**
Problems should only appear for real files.

**Screenshots**
See above.

**Extension Version**
`v0.25.7`

**Additional context**

Start by investigating how the extension contributes diagnostics to VS Code's Problems panel, using the reported `/commit~cf17f9d0/.github/workflows/build.yml` resource as the example. Compare behavior with the extension enabled and disabled, and determine how to verify that diagnostics are only shown for real, editable files.

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