[SPARK-56874][SQL] Fast-path DateTimeUtils.daysToMicros for ZoneOffset.UTC

[LuciferYang]

What changes were proposed in this pull request?

Add a ZoneOffset.UTC fast path to DateTimeUtils.daysToMicros(days, zoneId):

if (zoneId eq ZoneOffset.UTC) {
  Math.multiplyExact(days.toLong, MICROS_PER_DAY)
} else {
  // existing LocalDate -> ZonedDateTime -> Instant path
}

For UTC the answer is simply days * MICROS_PER_DAY, so the slow path's three heap allocations (LocalDate, ZonedDateTime, Instant) are wasted.

Why are the changes needed?

daysToMicros(days, ZoneOffset.UTC) is on the per-row hot path of the vectorized parquet reader (DateToTimestampNTZUpdater and the rebase variants), the row-based parquet converter (ParquetRowConverter), the Avro reader (AvroDeserializer), and DATE -> TIMESTAMP Cast (interpreted + codegen). All of them pass the ZoneOffset.UTC singleton, so the reference-equality fast path triggers everywhere it matters.

ParquetVectorUpdaterBenchmark results will be regenerated via GitHub Actions.

Does this PR introduce any user-facing change?

No, pure optimization. Behavior is preserved for every input in Spark's valid DateType range. Both paths use Math.multiplyExact internally and overflow at the same |days| ~= 107M boundary with the same ArithmeticException, far outside any reachable input.

How was this patch tested?

New DateTimeUtilsSuite contract test pins down the UTC fast path:

• Asserts it agrees with a fixed-offset zone (Etc/GMT) path for a representative set of days (zero, positive, negative, +/-maxSafeDays).
• Asserts it equals days * MICROS_PER_DAY directly, so divergence in some future JDK is caught.
• Asserts ArithmeticException on overflow.

Was this patch authored or co-authored using generative AI tooling?

No

[LuciferYang]

test first, will update benchmark results later

[acruise]

Just out of curiosity, is there a version of this that takes an arbitrary offset (e.g. in seconds) vs. a nominal zone ID? The distinction hardly ever matters, except when various jurisdictions change their timezone rules. For example, here in British Columbia, we have officially stopped doing DST changes! 😅