Filter out CSS002 when it appears in an "@@" (#11313)

Filters out the main user frustration arising from
#7349

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Diagnostics/CSSErrorCodes.cs

@@ -6,6 +6,7 @@ namespace Microsoft.CodeAnalysis.Razor.Diagnostics;
 // Note: This type should be kept in sync with WTE's ErrorCodes.cs
 internal static class CSSErrorCodes
 {
+    public const string UnrecognizedBlockType = "CSS002";
     public const string MissingOpeningBrace = "CSS023";
     public const string MissingSelectorAfterCombinator = "CSS029";
     public const string MissingSelectorBeforeCombinatorCode = "CSS031";

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Diagnostics/RazorTranslateDiagnosticsService.cs

@@ -234,6 +234,7 @@ private static bool ShouldFilterHtmlDiagnosticBasedOnErrorCode(LspDiagnostic dia
 
         return str switch
         {
+            CSSErrorCodes.UnrecognizedBlockType => IsEscapedAtSign(diagnostic, sourceText),
             CSSErrorCodes.MissingOpeningBrace => IsCSharpInStyleBlock(diagnostic, sourceText, syntaxTree),
             CSSErrorCodes.MissingSelectorAfterCombinator => IsCSharpInStyleBlock(diagnostic, sourceText, syntaxTree),
             CSSErrorCodes.MissingSelectorBeforeCombinatorCode => IsCSharpInStyleBlock(diagnostic, sourceText, syntaxTree),
@@ -244,6 +245,34 @@ private static bool ShouldFilterHtmlDiagnosticBasedOnErrorCode(LspDiagnostic dia
             _ => false,
         };
 
+        static bool IsEscapedAtSign(LspDiagnostic diagnostic, SourceText sourceText)
+        {
+            // Filters out "Unrecognized block type" errors in CSS, which occur with something like this:
+            //
+            // <style>
+            //     @@font - face
+            //     {
+            //         // contents
+            //     }
+            // </style>
+            //
+            // The "@@" tells Razor that the user wants an "@" in the final Html, but the design time document
+            // for the Html has to line up with the source Razor file, so that doesn't happen in the IDE. When
+            // CSS gets the two "@"s, it raises the "Unrecognized block type" error.
+
+            if (!sourceText.TryGetAbsoluteIndex(diagnostic.Range.Start, out var absoluteIndex))
+            {
+                return false;
+            }
+
+            // It's much easier to just check the source text directly, rather than try to understand all of the
+            // possible shapes of the syntax tree here. We assume that since the diagnostics we're filtering out
+            // came from the CSS server, it's a CSS block.
+            return absoluteIndex > 0 &&
+                sourceText[absoluteIndex] == '@' &&
+                sourceText[absoluteIndex - 1] == '@';
+        }
+
         static bool IsCSharpInStyleBlock(LspDiagnostic diagnostic, SourceText sourceText, RazorSyntaxTree syntaxTree)
         {
             // C# in a style block causes diagnostics because the HTML background document replaces C# with "~"

src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentPullDiagnosticsTest.cs

@@ -6,6 +6,7 @@
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor;
 using Microsoft.AspNetCore.Razor.Test.Common;
+using Microsoft.CodeAnalysis.Razor.Diagnostics;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.LanguageServer.Protocol;
 using Roslyn.Test.Utilities;
@@ -65,6 +66,47 @@ public Task Html()
             }]);
     }
 
+    [Fact]
+    public Task FilterEscapedAtFromCss()
+    {
+        TestCode input = """
+            <div>
+
+            <style>
+              @@media (max-width: 600px) {
+                body {
+                  background-color: lightblue;
+                }
+              }
+
+              {|CSS002:f|}oo
+              {
+                bar: baz;
+              }
+            </style>
+
+            </div>
+            """;
+
+        return VerifyDiagnosticsAsync(input,
+            htmlResponse: [new VSInternalDiagnosticReport
+            {
+                Diagnostics =
+                [
+                    new Diagnostic
+                    {
+                        Code = CSSErrorCodes.UnrecognizedBlockType,
+                        Range = SourceText.From(input.Text).GetRange(new TextSpan(input.Text.IndexOf("@@") + 1, 1))
+                    },
+                    new Diagnostic
+                    {
+                        Code = CSSErrorCodes.UnrecognizedBlockType,
+                        Range = SourceText.From(input.Text).GetRange(new TextSpan(input.Text.IndexOf("f"), 1))
+                    }
+                ]
+            }]);
+    }
+
     [Fact]
     public Task CombinedAndNestedDiagnostics()
         => VerifyDiagnosticsAsync("""