Add VS hidden-in-editor tag for Unnecessary diagnostics and cover with manual fourslash tests

[Copilot]

This addresses the review comment requesting coverage for VS-specific diagnostic tag conversion. The change ensures Unnecessary diagnostics include the VS extension tag when VS extensions are enabled, while preserving non-VS behavior.

• Diagnostic tag conversion

  • Updated internal/ls/lsconv/converters.go to append the VS hidden-in-editor tag (DiagnosticTag(2147483641)) when:
    • the diagnostic reports Unnecessary, and
    • client capabilities indicate Visual Studio extensions support.

• Manual fourslash coverage (requested)

  • Added internal/fourslash/tests/manual/unnecessarySuggestionVsDiagnosticTags_test.go with two focused cases:
    • non-VS capabilities: emits only DiagnosticTagUnnecessary
    • VS capabilities: emits DiagnosticTagUnnecessary + VS hidden-in-editor tag

if diagnostic.ReportsUnnecessary() && slices.Contains(opts.tagValueSet, lsproto.DiagnosticTagUnnecessary) {
    tags = append(tags, lsproto.DiagnosticTagUnnecessary)
    if opts.visualStudio {
        tags = append(tags, diagnosticTagVsHiddenInEditor) // DiagnosticTag(2147483641)
    }
}