Custom validations are not called in connection forms

### Under which category would you file this issue?

Airflow Core

### Apache Airflow version

3.2.0

### What happened and how to reproduce it?

In airflow 3.2.0, defining custom validation for connection form doesn't work correctly. Validation is not called

as a example :
```python
@classmethod
def get_connection_form_widgets(cls) -> dict[str, Any]:
"""Return connection widgets to add to connection form."""
from flask_appbuilder.fieldwidgets import BS3TextFieldWidget
from flask_babel import lazy_gettext
from flask import flash
from wtforms.validators import Optional, ValidationError

def validate_cms_authentication(form, field):
valid = {"secEnterprise", "secLDAP", "secWinAD", "secSAPR3"}
if field.data and field.data not in valid:
flash("valeur non valide", "error")
raise ValidationError(
"Valeur invalide. Choisir parmi : secEnterprise, secLDAP, secWinAD, secSAPR3"
)
from wtforms import StringField

return {
"cms_system": StringField(
lazy_gettext("Système"), widget=BS3TextFieldWidget()
),
"cms_authentication": StringField(
lazy_gettext("Authentification"),
description="Méthode d'authentication (secEnterprise, secLDAP, secWinAD, secSAPR3)",
widget=BS3TextFieldWidget(),
default="secEnterprise",
validators=[Optional(), validate_cms_authentication],
),
}
```
when saving this:

it is saved without error, I tried to add logging to check if validate_cms_authentication was called but no trace in log so I suppose that validation is not called

### What you think should happen instead?

Validation error messages should be displayed

### Operating System

_No response_

### Deployment

None

### Apache Airflow Provider(s)

_No response_

### Versions of Apache Airflow Providers

_No response_

### Official Helm Chart version

Not Applicable

### Kubernetes Version

_No response_

### Helm Chart configuration

_No response_

### Docker Image customizations

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start from the connection form handling in Airflow 3.2.0 and trace how the validators returned by get_connection_form_widgets are applied when the form is saved. Reproduce the cms_authentication example with an invalid value; done when the ValidationError message is displayed and valid values continue to save.

Written by the indexing model from the issue text.