`timestamp_seconds` falls back to Spark for decimal, byte and short input

[andygrove]

Part of #5572.

timestamp_seconds accepts only int, long, float and double, and falls the whole projection back to Spark for anything else:

case IntegerType | LongType | FloatType | DoubleType => Compatible()
case dt => Unsupported(Some(s"timestamp_seconds does not support input type $dt"))

spark/src/main/scala/org/apache/comet/serde/datetime.scala:495-496

The serde's own getUnsupportedReasons spells out what that leaves behind: "DecimalType, ByteType, and ShortType fall back to Spark."

The decimal case is the one that matters. timestamp_seconds(decimal_col) is how you get sub-microsecond-safe epoch conversion in Spark, and SecondsToTimestamp.doGenCode has a dedicated decimal branch that does the rounding Spark's semantics require. DecimalType, ByteType and ShortType are all in CometBatchKernelCodegen.isSupportedDataType.

Fix: mix CodegenDispatchFallback into CometSecondsToTimestamp. It currently extends CometScalarFunction[SecondsToTimestamp], which is fine — the mixin composes with it, as CometConcat and CometMapFromEntries already demonstrate.

[0lai0]

take