[SPARK-57184][SQL] Recurse into CalendarInterval children when appending a null struct

[MaxGekk]

What changes were proposed in this pull request?

Include CalendarIntervalType in the recursion guard of WritableColumnVector.appendStruct(boolean isNull), so that appending a NULL parent struct cascades appendStruct(true) into a CalendarInterval child column and advances all three of its grandchild columns (months / days / microseconds).

for (WritableColumnVector c: childColumns) {
  if (c.type instanceof StructType || c.type instanceof VariantType
      || c.type instanceof CalendarIntervalType) {
    c.appendStruct(true);
  } else {
    c.appendNull();
  }
}

This was split out of the nanosecond-timestamp ColumnVector PR (SPARK-57100, #56198) per review, since it is an independent fix.

Why are the changes needed?

A CalendarInterval column is struct-shaped: it is backed by three grandchild primitive columns (months as int, days as int, microseconds as long). The recursion guard in appendStruct only handled StructType and VariantType, so an interval child column took the else branch (c.appendNull()), which advances only the interval column's own cursor and leaves its three grandchild cursors un-advanced.

As a result, for a struct column with a CalendarInterval field, appending a NULL parent row left the interval's grandchild cursors behind by one. A subsequent non-null row then wrote its months/days/microseconds into the wrong (earlier) grandchild slots, and reading that row back returned a skewed value - silent data corruption for the nested struct-of-interval case.

Does this PR introduce any user-facing change?

Yes. Reading back a struct-of-interval column that contains a NULL parent row followed by a non-null row now returns the correct interval value instead of a skewed one. Previously it returned corrupted data.

How was this patch tested?

Added a unit test to ColumnarBatchSuite that uses RowToColumnConverter to convert a null parent struct followed by non-null struct-of-interval rows, and verifies the interval values are read back correctly. The test fails without the fix and passes with it.

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

Generated-by: Claude Opus 4.8

[MaxGekk]

Merging to master/4.x. Thank you @peter-toth for review.