fix(admin): render boolean fields under wtforms 3.2

[galuszkm]

Description

wtforms 3.2 removed the validation_attrs attribute from the base Input widget and moved it onto each concrete subclass. sqladmin's BooleanInputWidget subclasses widgets.Input directly and never defines its own validation_attrs, so under wtforms 3.2 the render path if k in self.validation_attrs raises AttributeError

This PR patches this issue.

Related Issues

N/A

Type of Change

• Bug fix

Testing

How have you tested the change?

• I ran uv run just check (lint + type check)
• I ran uv run just test for overall testing
• I added or updated tests that prove my fix is effective or my feature works
• I verified existing examples in examples/ still work

Checklist

• I have read the CONTRIBUTING document
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

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