Support nested schema references inside Lists (e.g., List<NestedModel>)

[tilucasoli]

Feature Request

Add support for nested schema references inside Ack.list() in the code generator.

Context

Currently, when using a List with a nested custom type like List<Address>, the generator cannot reference the nested schema. This was already acknowledged as a future feature in example/lib/schema_types_edge_cases.dart (lines 126-133):

/// EXPECTED BEHAVIOR (future):
/// - addresses: List<AddressType> or List<Map<String, dynamic>>
@AckType()
final contactListSchema = Ack.object({
  'name': Ack.string(),
  // Future: List of nested schema references
  // 'addresses': Ack.list(addressSchema),
});

Use Case

For genUI integration (and similar use cases), we need to define schemas with lists of complex nested objects:

@AckType()
final addressSchema = Ack.object({
  'street': Ack.string(),
  'city': Ack.string(),
  'zipCode': Ack.string(),
  'country': Ack.string(),
});

/// Schema that references another schema variable
///
/// EXPECTED BEHAVIOR:
/// - address field should NOT be null/missing
/// - Should generate: AddressType get address (or Map<String, dynamic>)
@AckType()
final personSchema = Ack.object({
  'name': Ack.string(),
  'email': Ack.string(),
  'address': addressSchema, // Reference to another schema
  'age': Ack.integer(),
});

Expected Behavior

The generator should produce:

extension type PersonType(Map<String, Object?> _data)
    implements Map<String, Object?> {
  static PersonType parse(Object? data) {
    final validated = personSchema.parse(data);
    return PersonType(validated as Map<String, Object?>);
  }

  static SchemaResult<PersonType> safeParse(Object? data) {
    final result = personSchema.safeParse(data);
    return result.match(
      onOk: (validated) =>
          SchemaResult.ok(PersonType(validated as Map<String, Object?>)),
      onFail: (error) => SchemaResult.fail(error),
    );
  }

  String get name => _data['name'] as String;

  String get email => _data['email'] as String;

  AddressType get address => AddressType.parse(_data['address']);

  int get age => _data['age'] as int;

  PersonType copyWith({
    String? name,
    String? email,
    Map<String, dynamic>? address,
    int? age,
  }) {
    return PersonType.parse({
      'name': name ?? this.name,
      'email': email ?? this.email,
      'address': address ?? this.address,
      'age': age ?? this.age,
    });
  }
}

Technical Details

In packages/ack_generator/lib/src/builders/field_builder.dart, the _buildListSchema method (lines 95-107) currently only handles primitive types properly. It needs to be extended to detect when the list item type is a custom model with its own schema and reference that schema variable.

Impact

This is a blocker for complex AI/LLM integrations that require nested data structures in their schemas.