[release/9.4] Add user-friendly error handling for aspire new when directory contains existing files (#10550)

* Initial plan

* Add user-friendly error handling for exit code 73 in aspire new command

Co-authored-by: mitchdenny <[email protected]>

* Add comprehensive test for ProjectAlreadyExistsException in DotNetCliRunner

Co-authored-by: mitchdenny <[email protected]>

* Replace exception-based flow control with direct exit code check for project creation conflicts

- Remove ProjectAlreadyExistsException from DotNetCliRunner.NewProjectAsync
- Add direct exit code 73 check in DotNetTemplateFactory.ApplyTemplateAsync with descriptive comment
- Update unit test to verify exit code 73 is returned instead of expecting exception
- All 112 CLI tests pass, maintaining full backward compatibility

Co-authored-by: mitchdenny <[email protected]>

* Update error message to be more accurate about file conflicts

Co-authored-by: davidfowl <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: mitchdenny <[email protected]>
Co-authored-by: davidfowl <[email protected]>

Author: 4 people

src/Aspire.Cli/Exceptions/ProjectAlreadyExistsException.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Aspire.Cli.Exceptions;
+
+/// <summary>
+/// Exception thrown when attempting to create a project in a directory that already contains files from a previous project.
+/// </summary>
+internal sealed class ProjectAlreadyExistsException : Exception
+{
+    public ProjectAlreadyExistsException()
+        : base("The output folder already contains files from a previous project.")
+    {
+    }
+
+    public ProjectAlreadyExistsException(string message)
+        : base(message)
+    {
+    }
+
+    public ProjectAlreadyExistsException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}

src/Aspire.Cli/Resources/TemplatingStrings.Designer.cs

@@ -217,4 +217,7 @@
   <data name="NoTemplateVersionsFound" xml:space="preserve">
     <value>No template versions were found. Please check your internet connection or NuGet source configuration.</value>
   </data>
+  <data name="ProjectAlreadyExists" xml:space="preserve">
+    <value>The new project cannot be created because it would overwrite existing files in the output folder. Please remove or move the conflicting files, or choose a different location.</value>
+  </data>
 </root>