fix casing mismatch when using non-unc file paths

Author: dibarbet

src/LanguageServer/Protocol/Handler/Diagnostics/AbstractDocumentPullDiagnosticHandler.cs

@@ -34,16 +34,16 @@ protected override ValueTask<ImmutableArray<IDiagnosticSource>> GetOrderedDiagno
         //
         // Only consider open documents here (and only closed ones in the WorkspacePullDiagnosticHandler).  Each
         // handler treats those as separate worlds that they are responsible for.
-        var textDocument = context.TextDocument;
-        if (textDocument is null)
+        var identifier = GetTextDocumentIdentifier(diagnosticsParams);
+        if (identifier is null || context.Document is null)
         {
             context.TraceDebug("Ignoring diagnostics request because no text document was provided");
             return new([]);
         }
 
-        if (!context.IsTracking(textDocument.GetURI()))
+        if (!context.IsTracking(identifier.DocumentUri))
         {
-            context.TraceWarning($"Ignoring diagnostics request for untracked document: {textDocument.GetURI()}");
+            context.TraceWarning($"Ignoring diagnostics request for untracked document: {identifier.DocumentUri}");
             return new([]);
         }