[SPARK-59507][SQL] Parse interval fractional seconds without building a padded string

`IntervalUtils.parseNanos` right-padded the fractional-second digits to 9 characters with a
string concatenation and a substring before parsing:

    (nanos + "000000000").substring(0, maxNanosLen)

That allocated two throwaway strings per call purely to zero-pad ahead of an integer parse.

Since the fractional part is guaranteed to be 1-9 ASCII digits by the interval grammar,
parse it directly and scale by the corresponding power of ten instead. The result is
identical -- the parsed value stays in [0, 999999999], so the range check and error
behavior are unchanged -- and no intermediate string is allocated.

Co-authored-by: Isaac <no-reply@databricks.com>

Author: david-mollitor-db

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/IntervalUtils.scala

@@ -540,14 +540,20 @@ object IntervalUtils extends SparkIntervalUtils {
     }
   }
 
+  // 10^0 .. 10^8: scales a 1..9 digit fraction up to nanosecond precision without
+  // building a zero-padded temporary string.
+  private val nanosMultiplier: Array[Long] =
+    Array(1L, 10L, 100L, 1000L, 10000L, 100000L, 1000000L, 10000000L, 100000000L)
+
   // Parses a string with nanoseconds, truncates the result and returns microseconds
   private def parseNanos(nanos: String, isNegative: Boolean): Long = {
     if (nanos != null) {
       val maxNanosLen = 9
-      val alignedStr = if (nanos.length < maxNanosLen) {
-        (nanos + "000000000").substring(0, maxNanosLen)
-      } else nanos
-      val nanoSecond = toLongWithRange(nanosStr, alignedStr, 0L, 999999999L)
+      // `nanos` is 1..9 digits (per the interval grammar), so parse it directly and scale
+      // by the missing trailing zeros instead of building a padded string. `raw` is in
+      // [0, 999999999], and scaling keeps it within that range.
+      val raw = toLongWithRange(nanosStr, nanos, 0L, 999999999L)
+      val nanoSecond = raw * nanosMultiplier(maxNanosLen - nanos.length)
       val microSecond = nanoSecond / NANOS_PER_MICROS
       if (isNegative) -microSecond else microSecond
     } else {