Show specific error message for dotnet <app_path> where app_path is a non-dll/exe file that exists (#116940)

Make running `dotnet <path>` where path exists but is not a dll/exe show "The application '<path>' does not exist or is not a managed .dll" and avoid invoking the SDK.

Author: Copilot

src/installer/tests/HostActivation.Tests/DotnetArgValidation.cs

@@ -68,6 +68,41 @@ public void MissingArgumentValue_Fails()
                 .And.HaveStdErrContaining($"Failed to parse supported options or their values:");
         }
 
+        [Fact]
+        public void File_ExistsNoDirectorySeparator_RoutesToSDK()
+        {
+            // Create a file named "build" in the current directory to simulate a file that happens to match the name of a command
+            string buildFile = Path.Combine(sharedTestState.BaseDirectory.Location, "build");
+            File.WriteAllText(buildFile, string.Empty);
+
+            // Test that "dotnet build" still routes to SDK, not to the file
+            TestContext.BuiltDotNet.Exec("build")
+                .WorkingDirectory(sharedTestState.BaseDirectory.Location)
+                .EnableTracingAndCaptureOutputs()
+                .Execute()
+                .Should().Fail()
+                .And.HaveStdErrContaining("The command could not be loaded, possibly because:") // This is a generic error for when we can't tell what exactly the user intended to do
+                .And.HaveStdErrContaining("Resolving SDKs");
+        }
+
+        [Fact]
+        public void RelativePathToNonManagedFile_ShowsSpecificError()
+        {
+            // Test relative path with directory separator
+            Directory.CreateDirectory(Path.Combine(sharedTestState.BaseDirectory.Location, "subdir"));
+            string testFile = Path.Combine(sharedTestState.BaseDirectory.Location, "subdir", "test.json");
+            File.WriteAllText(testFile, "{}");
+
+            string relativePath = Path.GetRelativePath(sharedTestState.BaseDirectory.Location, testFile);
+            TestContext.BuiltDotNet.Exec(relativePath)
+                .WorkingDirectory(sharedTestState.BaseDirectory.Location)
+                .CaptureStdOut()
+                .CaptureStdErr()
+                .Execute()
+                .Should().Fail()
+                .And.HaveStdErrContaining($"The application '{relativePath}' is not a managed .dll.");
+        }
+
         [Fact]
         public void InvalidFileOrCommand_NoSDK_ListsPossibleIssues()
         {

src/installer/tests/HostActivation.Tests/FrameworkDependentAppLaunch.cs

@@ -58,7 +58,7 @@ public void Muxer_AssemblyWithDifferentFileExtension_Fails()
                 .CaptureStdErr()
                 .Execute()
                 .Should().Fail()
-                .And.HaveStdErrContaining($"The application '{appOtherExt}' does not exist or is not a managed .dll or .exe");
+                .And.HaveStdErrContaining($"The application '{appOtherExt}' is not a managed .dll.");
         }
 
         [Fact]

src/native/corehost/fxr/command_line.cpp

@@ -164,6 +164,17 @@ namespace
                 trace::verbose(_X("Application '%s' is not a managed executable."), app_candidate.c_str());
                 if (!exec_mode)
                 {
+                    // Check if this is a non-managed file with directory separator that exists
+                    // This should show a specific error instead of routing to CLI
+                    bool has_dir_separator = app_candidate.find(DIR_SEPARATOR) != pal::string_t::npos;
+                    if (has_dir_separator)
+                    {
+                        if (pal::file_exists(app_candidate))
+                        {
+                            trace::error(_X("The application '%s' is not a managed .dll."), app_candidate.c_str());
+                            return StatusCode::InvalidArgFailure;
+                        }
+                    }
                     // Route to CLI.
                     return StatusCode::AppArgNotRunnable;
                 }