Disallow quotes in file-level directives (#51119)

Author: jjonescz

src/Cli/dotnet/Commands/CliCommandStrings.resx

@@ -1620,6 +1620,9 @@ Tool '{1}' (version '{2}') was successfully installed. Entry is added to the man
     <value>Duplicate directives are not supported: {0}</value>
     <comment>{0} is the directive type and name.</comment>
   </data>
+  <data name="QuoteInDirective" xml:space="preserve">
+    <value>Directives currently cannot contain double quotes (").</value>
+  </data>
   <data name="InvalidOptionForStdin" xml:space="preserve">
     <value>Cannot specify option '{0}' when also using '-' to read the file from standard input.</value>
     <comment>{0} is an option name like '--no-build'.</comment>

src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs

@@ -1505,8 +1505,15 @@ public static ImmutableArray<CSharpDirective> FindDirectives(SourceFile sourceFi
                     Diagnostics = diagnostics,
                     SourceFile = sourceFile,
                     DirectiveKind = name.ToString(),
-                    DirectiveText = value.ToString()
+                    DirectiveText = value.ToString(),
                 };
+
+                // Block quotes now so we can later support quoted values without a breaking change. https://github.com/dotnet/sdk/issues/49367
+                if (value.Contains('"'))
+                {
+                    diagnostics.AddError(sourceFile, context.Info.Span, CliCommandStrings.QuoteInDirective);
+                }
+
                 if (CSharpDirective.Parse(context) is { } directive)
                 {
                     // If the directive is already present, report an error.