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_

まず BooleanExpression、AlwaysTrue、AlwaysFalse を見つけ、次に issue で言及されている optional-expression のチェックと REST の residual/filter 処理を調べます。定数式だけが Python の truthiness をサポートし、非定数式はそれを拒否し、明示的な AlwaysFalse() 値が欠落値として扱われずに保持されることを確認してください。

索引モデルが issue の本文から書いたものです。