Document custom form validation via Callback rule in Forms guide

[Copilot]

The Forms guide did not explain how to perform custom validation beyond built-in attribute rules. This PR adds explicit guidance for callback-based validation directly in src/guide/start/forms.md.

• What was missing

  • The guide covered standard attribute rules (Length, etc.) but not custom verification logic in form models.
  • Follow-up examples did not reflect the newly introduced yaml field consistently.

• What was added

  • Extended the existing Creating a form example with callback-based custom validation.
  • Demonstrates #[Callback(method: ...)] on a form property.
  • Shows a custom validator method returning Yiisoft\Validator\Result.
  • Uses a YAML field example (yaml + validateYaml()) aligned with the validator callback rule documentation.
  • Adds an explicit Label attribute for the YAML field in the form model example.
  • Keeps callback guidance in the main form snippet (no separate custom-validation section).
  • Links to the validator package’s callback rule reference for deeper usage/signatures.
  • Updated further tutorial examples to use the new yaml field consistently (form rendering and explanatory flow), so examples remain aligned with the updated form model.
  • Updated the form rendering example to use Field::textarea($form, 'yaml') for YAML input in follow-up snippets.

• Example from the added docs

  use Exception;
  use Yiisoft\FormModel\FormModel;
  use Yiisoft\Validator\Label;
  use Yiisoft\Validator\Result;
  use Yiisoft\Validator\Rule\Callback;
  use Yiisoft\Validator\Rule\Length;
  
  final class Form extends FormModel
  {
      #[Label('The message to be echoed')]
      #[Length(min: 2)]
      public string $message = '';
  
      #[Label('YAML content')]
      #[Callback(method: 'validateYaml')]
      public string $yaml = '';
  
      private function validateYaml(mixed $value): Result
      {
          if (!is_string($value)) {
              return (new Result())->addError('The value must be a string.');
          }
  
          $notYamlMessage = 'This value is not valid YAML.';
  
          try {
              $data = yaml_parse($value);
          } catch (Exception) {
              return (new Result())->addError($notYamlMessage);
          }
  
          if ($data === false) {
              return (new Result())->addError($notYamlMessage);
          }
  
          return new Result();
      }
  }

[vjik]

@copilot merge it with main form example and use yaml field as in https://github.com/yiisoft/validator/blob/master/docs/guide/en/built-in-rules-callback.md

[Copilot]

@copilot merge it with main form example and use yaml field as in https://github.com/yiisoft/validator/blob/master/docs/guide/en/built-in-rules-callback.md

Updated in c38e01b: the callback section is now merged with the main Form example, and the custom callback uses a yaml field with validateYaml() (aligned with validator docs). Screenshot of the updated page: /tmp/forms-callback.png.

[vjik]

@copilot don't create new section. Add new field to form example in "Creating a form" section

[Copilot]

Done in 637fe1f: removed the separate section and moved the YAML Callback field into the existing “Creating a form” example. Screenshot: /tmp/forms-creating-form-merged.png.

[vjik]

@copilot Update further examples using the new field in the form.

[Copilot]

@copilot merge it with main form example and use yaml field as in https://github.com/yiisoft/validator/blob/master/docs/guide/en/built-in-rules-callback.md

Done in 637fe1f: the YAML callback validation is merged into the main Form example in the “Creating a form” section. Screenshot: /tmp/forms-with-yaml-field-updated.png.

@copilot Update further examples using the new field in the form.

Done in 106255a: I updated the further examples to use the new yaml field in the form rendering and usage flow. Screenshot: /tmp/forms-with-yaml-field-updated.png.

[vjik]

@copilot Use textarea

[Copilot]

Done in d98eadc: switched the yaml field example to use a textarea and updated related wording accordingly. Screenshot: /tmp/forms-with-yaml-textarea-updated.png.

[vjik]

@copilot Add label attribute

[Copilot]

Done in d767eff: added a Label attribute to the yaml field in the main form example. Screenshot: /tmp/forms-label-attribute-updated.png.

[Copilot]

@copilot merge it with main form example and use yaml field as in https://github.com/yiisoft/validator/blob/master/docs/guide/en/built-in-rules-callback.md

Addressed in 637fe1f: the callback example is merged into the main “Creating a form” example and uses the yaml field with validateYaml() aligned with validator docs. Screenshot: /tmp/forms-label-attribute-updated.png.