fix(gendoc): accept .slnx solutions in the documentation generator

FirstClassErrors.GenDoc.UnitTests/SolutionErrorDocumentationGeneratorTests.cs

@@ -46,6 +46,29 @@ public void GetErrorDocumentationFromRejectsANonSolutionFile() {
         }
     }
 
+    [Theory(DisplayName = "GetErrorDocumentationFrom accepts the .sln and .slnx solution formats.")]
+    [InlineData(".sln")]
+    [InlineData(".slnx")]
+    public void GetErrorDocumentationFromAcceptsSolutionFormats(string extension) {
+        // Setup: a real file carrying a solution extension. It is not a valid solution, so the enumeration further
+        // down the pipeline will ultimately fail — but only *after* the extension validation, which is what this test
+        // guards. The rejected-extension path is the one that throws ArgumentException (see the sibling test); proving
+        // the format is accepted therefore means the call throws anything *but* an ArgumentException.
+        string path = Path.ChangeExtension(Path.GetTempFileName(), extension);
+        File.WriteAllText(path, string.Empty);
+
+        try {
+            // Exercise
+            Exception? caught = Record.Exception(
+                () => SolutionErrorDocumentationGenerator.GetErrorDocumentationFrom(path, new SolutionGenerationOptions { BuildSolution = false }).ToList());
+
+            // Verify: the extension was accepted, so no ArgumentException was raised for it.
+            Check.That(caught).IsNotInstanceOf<ArgumentException>();
+        } finally {
+            File.Delete(path);
+        }
+    }
+
     [Fact(DisplayName = "GetErrorDocumentationFromAssemblies rejects a null path list.")]
     public void GetErrorDocumentationFromAssembliesRejectsANullPathList() {
         // Exercise & verify

FirstClassErrors.GenDoc/SolutionErrorDocumentationGenerator.cs

@@ -31,7 +31,14 @@ public static IEnumerable<ErrorDocumentation> GetErrorDocumentationFrom(string s
 
         string fullSolutionPath = Path.GetFullPath(solutionPath);
         if (File.Exists(fullSolutionPath) is false) { throw new FileNotFoundException($"Solution file not found: '{fullSolutionPath}'", fullSolutionPath); }
-        if (string.Equals(Path.GetExtension(fullSolutionPath), ".sln", StringComparison.OrdinalIgnoreCase) is false) { throw new ArgumentException($"Expected a .sln file path, got: '{fullSolutionPath}'", nameof(solutionPath)); }
+
+        // Accept both the classic (.sln) and the XML (.slnx) solution formats: "dotnet sln list", used below to
+        // enumerate the projects, handles the two uniformly. Solution filters (.slnf) are intentionally excluded —
+        // the "dotnet sln" subcommand does not process them.
+        string extension = Path.GetExtension(fullSolutionPath);
+        bool isSolution = string.Equals(extension, ".sln", StringComparison.OrdinalIgnoreCase)
+                       || string.Equals(extension, ".slnx", StringComparison.OrdinalIgnoreCase);
+        if (isSolution is false) { throw new ArgumentException($"Expected a .sln or .slnx file path, got: '{fullSolutionPath}'", nameof(solutionPath)); }
 
         if (options.BuildSolution) {
             DotNetBuild(fullSolutionPath, options);