fix: prevent linting of deleted temp files

[toscm]

This PR fixes stale diagnostics being kept for temporary/deleted documents (for example git diff/revision views), which could accumulate in the 'Problems' panel as path[1]="": No such file or directory errors. See issue #1630 for details (including a video and screenshot).

Changes:

• Drop diagnostics when the URI has an empty file path.
• Drop diagnostics for unsupported URI schemes.
• Drop diagnostics for file URIs that no longer exist.
• Restrict single-server document selector to supported schemes only, e.g.:
  • file:///Users/me/project/script.R => selected and linted
  • untitled:Untitled-1 => selected and linted
  • vscode-notebook-cell:/... => selected and linted
  • git:/... (diff/revision temp document) => not selected
  • other temporary/non-standard schemes => not selected

Validation:

• Reproduced with repeated open/click/close of git diff temporary revisions on my local machine (VSCode=1.109.5, R=4.5.2, MacOS=26.2, Model=MacBook Pro M2 Max)
• Confirmed stale missing-path diagnostics no longer accumulate.
• Confirmed real diagnostics for existing files are still shown correctly.
• Packaged and installed VSIX for manual verification (available for download at toscm/vscode-R/releases/tag/v2.8.6-tempfix.1)