Fix flaky AllProjectsHaveSameOutputDirectory on Windows via delete-with-retry

[Copilot]

IncrementalBuildTest.AllProjectsHaveSameOutputDirectory intermittently fails on Windows with UnauthorizedAccessException : Access to the path 'App1.dll' is denied. SolutionBuilder.Save() creates a fresh ProjectBuilder per project on every call, so BuiltBefore == false and ProjectBuilder.Save() always takes the recursive-delete-and-repopulate branch — deleting a just-built App1.dll whose handle may still be held by VBCSCompiler, Defender, or the search indexer (opened without FILE_SHARE_DELETE).

Changes

• FileSystemUtils.DeleteDirectoryWithRetry — new helper that sets the directory writable and retries Directory.Delete(..., true) with linear back-off, catching only transient UnauthorizedAccessException/IOException. This neutralizes the race regardless of which process holds the handle, with no change to test semantics.

  public static void DeleteDirectoryWithRetry (string directory, int retries = 10)
  {
      if (!Directory.Exists (directory))
          return;
      SetDirectoryWriteable (directory);
      for (int i = 0; ; i++) {
          try {
              Directory.Delete (directory, true);
              return;
          } catch (Exception e) when ((e is UnauthorizedAccessException || e is IOException) && i < retries) {
              Thread.Sleep (200 * (i + 1)); // back off; let AV/Roslyn release the handle
              SetDirectoryWriteable (directory);
          }
      }
  }

• ProjectBuilder.Save / ProjectBuilder.Cleanup — replaced the inline SetDirectoryWriteable + Directory.Delete with the new helper.

• SolutionBuilder.Dispose — same recursive delete now routed through the helper (existing swallow-on-CI behavior retained).

The loop's final iteration (i >= retries) lets the original exception propagate, preserving fail-loud behavior if the lock is genuinely permanent.

[Copilot]

Pull request overview

This PR addresses intermittent Windows test failures in IncrementalBuildTest.AllProjectsHaveSameOutputDirectory by making test-directory cleanup more resilient to transient file locks (e.g., Roslyn shared compilation server, AV, or indexer) via a delete-with-retry helper in Xamarin.ProjectTools.

Changes:

• Added FileSystemUtils.DeleteDirectoryWithRetry() to retry recursive directory deletion on transient UnauthorizedAccessException/IOException with backoff.
• Updated ProjectBuilder.Save() and ProjectBuilder.Cleanup() to use the new helper instead of inline delete logic.
• Updated SolutionBuilder.Dispose() to route recursive solution directory cleanup through the helper (while retaining existing swallow-on-CI behavior).

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Utilities/FileSystemUtils.cs	Introduces DeleteDirectoryWithRetry for transient-lock tolerant recursive deletion.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/SolutionBuilder.cs	Uses the new retrying delete helper during solution cleanup in Dispose.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/ProjectBuilder.cs	Replaces direct recursive deletes with the retrying helper in Save and Cleanup.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 1

[simonrozsival]

@copilot break on DirectoryNotFoundException instead of retrying

[Copilot]

Updated in 848ccdf: DeleteDirectoryWithRetry() now treats DirectoryNotFoundException as success (returns immediately) instead of retrying.