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_

Start by locating BooleanExpression, AlwaysTrue, and AlwaysFalse, then inspect the optional-expression checks and REST residual/filter handling mentioned in the issue. Verify that only constant expressions support Python truthiness, non-constant expressions reject it, and explicit AlwaysFalse() values are preserved rather than treated as missing.

Written by the indexing model from the issue text.