perf: Adopt bulk-NULL builders and NullBuffer::union_many in datafusion-spark #22454

Is your feature request related to a problem or challenge?

Two patterns from @neilconway's recent perf PRs are not yet applied throughout the datafusion-spark crate:

1. Bulk-NULL builders (#21789, #21849, #21863, #21847, #21854, #21877, #21991, #22029, #21519, #21366) — when an output null mask equals (or is a NullBuffer::union[_many] of) the input null masks, precompute it once instead of maintaining it row-by-row with append_null / BooleanBufferBuilder.append.
2. NullBuffer::union_many (#22070) — replaces chains of 3+ binary NullBuffer::union calls with one clone + in-place &=.

An audit of the Spark crate turned up six sites where these patterns apply.

Describe the solution you'd like

Apply the patterns at the following call sites. Each checkbox is one PR.

• datafusion/spark/src/function/math/width_bucket.rs:206-276, 313-432 — width_bucket kernel has four nullable inputs (value, min, max, n_buckets). The macro generates the same code for Float64, Duration, IntervalYearMonth, IntervalMonthDayNano. Precompute NullBuffer::union_many([v.nulls(), min.nulls(), max.nulls(), n.nulls()]) outside the loop, append placeholders for masked rows, reattach at the end.
• datafusion/spark/src/function/array/slice.rs:152-186 — calculate_start_end runs two Int64Builders in lockstep, both append_null when any of (values, start, length) is null. Precompute union_many of three input nulls, fill placeholders, reattach to both builders.
• datafusion/spark/src/function/string/substring.rs:371-401 — SparkSubstring's main loop. Three-input null union (array + start_array + optional length_array). Negative-length branch still needs row-level append_empty, but the null mask itself can be precomputed.
• datafusion/spark/src/function/datetime/make_dt_interval.rs:170-197 — 1-4 nullable inputs wrapped in Option<&ArrayRef>. union_many over the Some(...) arrays' nulls.
• datafusion/spark/src/function/datetime/make_interval.rs:190- — same structure as make_dt_interval.
• datafusion/spark/src/function/string/format_string.rs:126-157 — accumulates Vec<String> then StringArray::from(vec). Switch to GenericStringBuilder + append_value(&str). Matches the append_with pattern from perf: Add append_with to string builders, use in replace #22029.
• datafusion/spark/src/function/conversion/cast.rs:199-209, 226-236 — cast_int_to_timestamp / cast_float_to_timestamp do per-row append_null / append_value where output nulls equal input nulls. Clone input nulls.

Describe alternatives you've considered

Bundling all sites into a single PR. Splitting per file keeps each PR reviewable, especially the width_bucket macro change which touches four type variants.

Additional context

Already use these patterns, skip:

• datafusion/spark/src/function/spark/null_utils.rs:63 — uses union_many.
• datafusion/spark/src/function/map/str_to_map.rs:212 — uses union_many.
• datafusion/spark/src/function/hash/utils.rs — fast-path null-count checks.

Considered and rejected:

• datafusion/spark/src/function/string/elt.rs:115-139 — output null mask is data-dependent (which column's nulls to include depends on the runtime idx value).