Negative-scale decimal /, div, ceil and floor fall back to Spark while round dispatches #7023

Part of #5572. Follow-up from #5050.

With spark.sql.legacy.allowNegativeScaleOfDecimal=true, a negative-scale decimal input falls the whole projection back to Spark for four serdes:

• CometCeil and CometFloor return Unsupported("Decimal type $t has negative scale") (math.scala#L44-L45, #L68-L69).
• Once fix: fall back to Spark for negative-scale decimal casts and arithmetic #5050 lands, CometDivide and CometIntegralDivide return Unsupported(negScaleDecimalArithmeticReason) from MathBase.negScaleDecimalRejection (arithmetic.scala#L105-L119). Their native kernel can't take a negative scale: decimal_div computes its scale-alignment exponents in u32, and fix: fall back to Spark for negative-scale decimal casts and arithmetic #5050 makes it reject one.

CometRound returns Unsupported for the same input but mixes in CodegenDispatchFallback, so round stays in the Comet pipeline and runs Spark's own doGenCode (arithmetic.scala#L366-L382). round on negative-scale decimal in CometExpressionSuite covers it. The other four could do the same. The EPIC deferred ceil and floor as marginal, since only the legacy flag reaches them, and #5050 adds division to that group.

Fix: for CometCeil and CometFloor the negative scale is the only Unsupported case, so mixing in CodegenDispatchFallback is the whole change. For CometDivide and CometIntegralDivide it isn't a one-line change. The mixin would also send their other Unsupported results to the dispatcher, namely mathDataTypeSupportLevel's unsupported-datatype case and CometDivide's check that a decimal result has decimal operands. Each of those needs checking against the dispatcher's type gate, or these serdes need a way to dispatch only the negative-scale case.

Tests should cover a materialized negative-scale column, not only a cast literal. That input broke the dispatcher's decimal reader until #5050 made it read negative scales through BigDecimal.