More robust solution for form validation

In the Spring Web MVC there are some helpers to improve the form data binding and validation.

1. In krazo, when the validation is failed, we have to **rebuild the from data** in the request. see my sample [TaskController.java#L94-L104](https://github.com/hantsy/jakartaee-mvc-sample/blob/master/src/main/java/com/example/web/TaskController.java#L94-L104), but in Spring WebMVC, it is not required, check [TaskController.java#L94-L97](https://github.com/hantsy/spring4-sandbox/blob/master/mvc-thymeleaf/src/main/java/com/hantsylabs/example/spring/web/TaskController.java#L94-L97).
2. In Spring MVC, there are some helpers for the views, including, JSP tags, Thymeleaf tags, Freemarker Marco, etc, to help to build the form more effectively. If there any helpers planned in Krazo do the same job or add some methods like `hasErrors(), getErrors()` in `MvcContext`.

Start by comparing the linked TaskController.java examples and reading the existing MvcContext API. Investigate how Krazo currently handles failed validation and view form data, then define the scope and acceptance criteria for validation helpers and view integration before implementing anything.

Written by the indexing model from the issue text.