fix: avoid bitmap range panic on inverted bounds

[fenfeng9]

closes #5878

Problem

BITMAP index panics when receiving inverted range queries like BETWEEN 2 AND 1 or x > 2 AND x < 2. This is because BTreeMap::range() panics when start > end or when both bounds are Excluded at the same value.

Solution

Add range validation matching BTREE's existing behavior:

• [a, b]: empty if a > b
• [a, b), (a, b], (a, b): empty if a >= b

Returns empty result for empty ranges instead of panicking.

[fenfeng9]

PTAL @wjones127

[wjones127]

Looks reasonable. I'm a little surprised those expressions are just simplified to false by the expression optimizer.