[BaseTasks] Fix NRE in DeleteFile catch block (#342)

Use pattern matching (`if (log is TaskLoggingHelper helper)`) instead of
an unconditional `as` cast + dereference. When `log` is null or not a
TaskLoggingHelper, the original deletion exception is now silently caught
instead of being masked by a NullReferenceException.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

src/Microsoft.Android.Build.BaseTasks/Files.cs

@@ -613,8 +613,9 @@ public static void DeleteFile (string filename, object log)
 			try {
 				File.Delete (filename);
 			} catch (Exception ex) {
-				var helper = log as TaskLoggingHelper;
-				helper.LogErrorFromException (ex);
+				if (log is TaskLoggingHelper helper) {
+					helper.LogErrorFromException (ex);
+				}
 			}
 		}
 

tests/Microsoft.Android.Build.BaseTasks-Tests/FilesTests.cs

@@ -726,5 +726,21 @@ public void ToHashString ()
 			var expected = BitConverter.ToString (bytes).Replace ("-", string.Empty);
 			Assert.AreEqual (expected, Files.ToHexString (bytes));
 		}
+
+		[Test]
+		public void DeleteFile_NullLog_DoesNotThrow ()
+		{
+			var path = Path.Combine (tempDir, "directory-instead-of-file");
+			Directory.CreateDirectory (path);
+			Assert.DoesNotThrow (() => Files.DeleteFile (path, null));
+		}
+
+		[Test]
+		public void DeleteFile_NonTaskLoggingHelperLog_DoesNotThrow ()
+		{
+			var path = Path.Combine (tempDir, "directory-instead-of-file-nontasklog");
+			Directory.CreateDirectory (path);
+			Assert.DoesNotThrow (() => Files.DeleteFile (path, "not a TaskLoggingHelper"));
+		}
 	}
 }