Deduplicate bucket computation in width_bucket_interval_mdn_exact #26307

Is your feature request related to a problem or challenge?

width_bucket_interval_mdn_exact (spark/src/function/math/width_bucket.rs#L300-L431) contains two near-identical blocks, "only month" (about L330-L366) and "months equals" (about L368-L424). Each repeats:

• the ascending/descending out-of-range checks (bucket 0 or num_buckets + 1),
• the bucket-width computation with its zero / non-finite null guard,
• the floor-plus-one bucket clamp.

They differ only in how x, l and h are projected to f64 (months vs. day/nanosecond offset relative to l). Duplicated numeric edge-case handling like this is where a boundary fix ends up in only one copy.

Describe the solution you'd like

Extract fn bucket_from_f64(x: f64, l: f64, h: f64, num_buckets: i64, ascending: bool) -> Option<i64> (returning None for null). Each branch then only computes its projected (x, l, h) and calls it. Optionally move the per-row body into a helper returning Option<i64> so the continue chains disappear. The existing width_bucket tests cover the behaviour.

Describe alternatives you've considered

None.

Additional context

This is small and self-contained, a good first issue. Line references are pinned to commit 0d79252.