perf(query): infer equality for identical singleton domains

[dantengsky]

Summary

SimpleDomain::domain_eq() currently returns Full for any overlapping domains. As a result, two identical singleton domains such as [x, x] and [x, x] remain Full even though equality is guaranteed to be true. SimpleDomain::domain_noteq() has the symmetric missed inference and remains Full instead of all-false.

The current behavior is conservative and correct. This is a low-priority optimization opportunity, not a correctness bug.

Expected refinement

• domain_eq([x, x], [x, x]) returns all-true.
• domain_noteq([x, x], [x, x]) returns all-false.
• Disjoint domains keep their existing exact results.
• Overlapping non-singleton domains remain Full because domain ranges do not preserve row-level correlation.

Scope

• Confirm the domain contract for singleton, empty, approximate, and unbounded domains before changing the generic comparison logic.
• Add coverage for numeric, string, and nullable inputs.
• Check the effect on constant folding, pruning, and selectivity estimation.

This was noticed while reviewing the LIKE domain work in #20207.