Add missing progress code sample files

- progress_upload_basic.dart (referenced in Quick Example)
- progress_multi_step.dart (referenced in Common Patterns)
- progress_batch_processing.dart (referenced in Common Patterns)
- progress_mock_testing.dart (referenced in Testing section)

These files were created earlier but not committed.

Author: escamoteur

code_samples/lib/command_it/progress_batch_processing.dart

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+import 'package:command_it/command_it.dart';
+import '_shared/stubs.dart';
+
+// #region example
+final batchCommand = Command.createAsyncWithProgress<List<Item>, String>(
+  (items, handle) async {
+    final total = items.length;
+    int processed = 0;
+
+    for (final item in items) {
+      if (handle.isCanceled.value) {
+        return 'Canceled ($processed/$total processed)';
+      }
+
+      await processItem(item);
+      processed++;
+
+      handle.updateProgress(processed / total);
+      handle.updateStatusMessage('Processed $processed of $total items');
+    }
+
+    return 'Processed $total items';
+  },
+  initialValue: '',
+);
+// #endregion example
+
+void main() {
+  // Example usage
+  final items = [Item(), Item(), Item()];
+  batchCommand.run(items);
+}

code_samples/lib/command_it/progress_mock_testing.dart

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+import 'package:command_it/command_it.dart';
+import 'dart:io';
+
+// #region example
+void testProgressUpdates() {
+  final mockCommand = MockCommand<File, String>(
+    initialValue: '',
+    withProgressHandle: true, // Enable progress simulation
+  );
+
+  // Simulate progress updates
+  mockCommand.updateMockProgress(0.0);
+  mockCommand.updateMockStatusMessage('Starting upload...');
+  assert(mockCommand.progress.value == 0.0);
+  assert(mockCommand.statusMessage.value == 'Starting upload...');
+
+  mockCommand.updateMockProgress(0.5);
+  mockCommand.updateMockStatusMessage('Uploading...');
+  assert(mockCommand.progress.value == 0.5);
+
+  mockCommand.mockCancel();
+  assert(mockCommand.isCanceled.value == true);
+
+  mockCommand.dispose();
+}
+// #endregion example
+
+void main() {
+  testProgressUpdates();
+}

code_samples/lib/command_it/progress_multi_step.dart

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+import 'package:command_it/command_it.dart';
+import '_shared/stubs.dart';
+
+// #region example
+final multiStepCommand = Command.createAsyncWithProgress<void, String>(
+  (_, handle) async {
+    // Step 1: Download (0-40%)
+    handle.updateStatusMessage('Downloading data...');
+    await downloadData();
+    handle.updateProgress(0.4);
+
+    // Step 2: Process (40-80%)
+    handle.updateStatusMessage('Processing data...');
+    await processData();
+    handle.updateProgress(0.8);
+
+    // Step 3: Save (80-100%)
+    handle.updateStatusMessage('Saving results...');
+    await saveResults();
+    handle.updateProgress(1.0);
+
+    return 'Complete';
+  },
+  initialValue: '',
+);
+// #endregion example
+
+void main() {
+  // Example usage
+  multiStepCommand.run();
+}

code_samples/lib/command_it/progress_upload_basic.dart

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+import 'package:command_it/command_it.dart';
+import 'dart:io';
+import '_shared/stubs.dart';
+
+// #region example
+final uploadCommand = Command.createAsyncWithProgress<File, String>(
+  (file, handle) async {
+    for (int i = 0; i <= 100; i += 10) {
+      if (handle.isCanceled.value) return 'Canceled';
+
+      await uploadChunk(file, i);
+      handle.updateProgress(i / 100.0);
+      handle.updateStatusMessage('Uploading: $i%');
+    }
+    return 'Complete';
+  },
+  initialValue: '',
+);
+// #endregion example
+
+void main() {
+  // Example usage
+  uploadCommand.run(File('example.txt'));
+}