fix(expression): treat constants as neutral in IsBoundVisitor

[YinZheng-Sun]

What

IsBoundVisitor previously returned an error for AlwaysTrue/AlwaysFalse,
so any expression tree that still contained a constant (e.g. built via
And::Make / Or::Make without folding) could not be checked.

This PR changes the visitor's internal result type to std::optional<bool>:

• std::nullopt means the subtree contains no references (True/False),
  so constants are neutral when combining binding states:
  And(True, unbound) is unbound and And(True, bound) is bound.
• Mixing bound and unbound predicates is now rejected with
  Found partially bound expression instead of silently returning false.
• The public IsBoundVisitor::IsBound() API still returns a definite bool
  (an expression with only constants is reported as not bound).

This follows Java's IsBoundVisitor.

Why

Split out from #956

Tests

• Updated IsBoundVisitorTest for constants and mixed binding states.
• Added AndWithConstants / OrWithConstants cases.

[wgtmac]

Thanks for separating a smaller PR! @YinZheng-Sun