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: <img width="796" height="76" alt="Image" src="https://github.com/user-attachments/assets/88b1999b-843b-4f24-8ca0-58ab9af32f1d" /> 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 ### Operati…