feat(postgrest): implement maxAffected method

Add maxAffected method to PostgrestTransformBuilder that sets the maximum
number of rows that can be affected by update and delete operations.

- Add maxAffected method to PostgrestTransformBuilder class
- Set handling=strict and max-affected={value} in Prefer header
- Preserve existing Prefer headers when adding maxAffected
- Support method chaining with other transform methods
- Add comprehensive unit and integration tests
- Documentation notes PostgREST v13+ requirement

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: grdsdev

packages/postgrest/lib/src/postgrest_transform_builder.dart

@@ -246,6 +246,38 @@ class PostgrestTransformBuilder<T> extends RawPostgrestBuilder<T, T, T> {
     return ResponsePostgrestBuilder(_copyWithType(headers: newHeaders));
   }
 
+  /// Sets the maximum number of rows that can be affected by the query.
+  ///
+  /// Only available with PATCH and DELETE operations. Requires PostgREST v13 or higher.
+  /// When the limit is exceeded, the query will fail with an error.
+  ///
+  /// ```dart
+  /// supabase.from('users').update({'active': false}).eq('status', 'inactive').maxAffected(5);
+  /// ```
+  ///
+  /// ```dart
+  /// supabase.from('users').delete().eq('active', false).maxAffected(10);
+  /// ```
+  PostgrestTransformBuilder<T> maxAffected(int value) {
+    final newHeaders = {..._headers};
+
+    // Add handling=strict and max-affected headers
+    if (newHeaders['Prefer'] != null) {
+      var preferHeader = newHeaders['Prefer']!;
+      if (!preferHeader.contains('handling=strict')) {
+        preferHeader += ',handling=strict';
+      }
+      if (!preferHeader.contains('max-affected=')) {
+        preferHeader += ',max-affected=$value';
+      }
+      newHeaders['Prefer'] = preferHeader;
+    } else {
+      newHeaders['Prefer'] = 'handling=strict,max-affected=$value';
+    }
+
+    return PostgrestTransformBuilder(_copyWith(headers: newHeaders));
+  }
+
   /// Obtains the EXPLAIN plan for this request.
   ///
   /// Before using this method, you need to enable `explain()` on your

packages/postgrest/test/transforms_test.dart

@@ -2,6 +2,7 @@ import 'package:collection/collection.dart';
 import 'package:postgrest/postgrest.dart';
 import 'package:test/test.dart';
 
+import 'custom_http_client.dart';
 import 'reset_helper.dart';
 
 void main() {
@@ -343,4 +344,139 @@ void main() {
     expect(res, isNotNull);
     expect(res['type'], 'FeatureCollection');
   });
+
+  group('maxAffected', () {
+    test('maxAffected method can be called on update operations', () {
+      expect(
+        () => postgrest
+            .from('users')
+            .update({'status': 'INACTIVE'})
+            .eq('id', 1)
+            .maxAffected(1),
+        returnsNormally,
+      );
+    });
+
+    test('maxAffected method can be called on delete operations', () {
+      expect(
+        () => postgrest
+            .from('channels')
+            .delete()
+            .eq('id', 999)
+            .maxAffected(5),
+        returnsNormally,
+      );
+    });
+
+    test('maxAffected method can be called on select operations', () {
+      expect(
+        () => postgrest.from('users').select().maxAffected(1),
+        returnsNormally,
+      );
+    });
+
+    test('maxAffected method can be called on insert operations', () {
+      expect(
+        () => postgrest
+            .from('users')
+            .insert({'username': 'test'})
+            .maxAffected(1),
+        returnsNormally,
+      );
+    });
+
+    test('maxAffected method can be chained with select', () {
+      expect(
+        () => postgrest
+            .from('users')
+            .update({'status': 'INACTIVE'})
+            .eq('id', 1)
+            .maxAffected(1)
+            .select(),
+        returnsNormally,
+      );
+    });
+  });
+
+  group('maxAffected integration', () {
+    late CustomHttpClient customHttpClient;
+    late PostgrestClient postgrestCustomHttpClient;
+
+    setUp(() {
+      customHttpClient = CustomHttpClient();
+      postgrestCustomHttpClient = PostgrestClient(
+        rootUrl,
+        httpClient: customHttpClient,
+      );
+    });
+
+    test('maxAffected sets correct headers for update', () async {
+      try {
+        await postgrestCustomHttpClient
+            .from('users')
+            .update({'status': 'INACTIVE'})
+            .eq('id', 1)
+            .maxAffected(5);
+      } catch (_) {
+        // Expected to fail with custom client, we just want to check headers
+      }
+
+      expect(customHttpClient.lastRequest, isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('handling=strict'));
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('max-affected=5'));
+    });
+
+    test('maxAffected sets correct headers for delete', () async {
+      try {
+        await postgrestCustomHttpClient
+            .from('users')
+            .delete()
+            .eq('id', 1)
+            .maxAffected(10);
+      } catch (_) {
+        // Expected to fail with custom client, we just want to check headers
+      }
+
+      expect(customHttpClient.lastRequest, isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('handling=strict'));
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('max-affected=10'));
+    });
+
+    test('maxAffected preserves existing Prefer headers', () async {
+      try {
+        await postgrestCustomHttpClient
+            .from('users')
+            .update({'status': 'INACTIVE'})
+            .eq('id', 1)
+            .select()
+            .maxAffected(3);
+      } catch (_) {
+        // Expected to fail with custom client, we just want to check headers
+      }
+
+      expect(customHttpClient.lastRequest, isNotNull);
+      final preferHeader = customHttpClient.lastRequest!.headers['Prefer']!;
+      expect(preferHeader, contains('return=representation'));
+      expect(preferHeader, contains('handling=strict'));
+      expect(preferHeader, contains('max-affected=3'));
+    });
+
+    test('maxAffected works with select operations (sets headers but likely ineffective)', () async {
+      try {
+        await postgrestCustomHttpClient
+            .from('users')
+            .select()
+            .maxAffected(2);
+      } catch (_) {
+        // Expected to fail with custom client, we just want to check headers
+      }
+
+      expect(customHttpClient.lastRequest, isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], isNotNull);
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('handling=strict'));
+      expect(customHttpClient.lastRequest!.headers['Prefer'], contains('max-affected=2'));
+    });
+  });
 }