Add explicit truthiness semantics for constant BooleanExpressions

While this PR can use expression negation via `__invert__,` I think it may be worth supporting Python truthiness for the constant expressions in a follow-up:
```
class AlwaysTrue:
def __bool__(self) -> bool:
return True

class AlwaysFalse:
def __bool__(self) -> bool:
return False
```

That would make patterns like this behave intuitively:
```
if not self.visit_starts_with(term, literal):
return AlwaysTrue()
```

When adding this, we should also be careful that Python truthiness is for control flow, not expression construction: `not expr` returns a Python `bool`, while `~expr` returns a negated BooleanExpression. It would probably be safest for non-constant `BooleanExpression.__bool__` to raise, with only `AlwaysTrue` and `AlwaysFalse` overriding it. The follow-up should also audit optional-expression checks like `expr or AlwaysTrue()` and REST residual/filter handling so explicit `AlwaysFalse()` values are not treated as missing.

_Originally posted by @kevinjqliu in https://github.com/apache/iceberg-python/issues/3503#issuecomment-4763176953_

Commencez par localiser BooleanExpression, AlwaysTrue et AlwaysFalse, puis examinez les vérifications des expressions optionnelles et la gestion des résidus/filtres REST mentionnées dans l’issue. Vérifiez que seules les expressions constantes prennent en charge l’évaluation de vérité de Python, que les expressions non constantes la refusent et que les valeurs explicites AlwaysFalse() sont conservées au lieu d’être traitées comme absentes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.