fix: fall back to Spark for negative-scale decimal casts and arithmetic

[0lai0]

Which issue does this PR close?

Closes #5013

Rationale for this change

With spark.sql.legacy.allowNegativeScaleOfDecimal=true, Spark allows DecimalType(p, s) with s < 0. Comet's native cast and decimal arithmetic kernels scale-align values by computing 10^|scale| (e.g. 10_i128.pow(scale as u32)). When scale is negative, casting it to u32 produces a huge value, so the power computation overflows: it panics (attempt to multiply/subtract with overflow) in debug builds and silently wraps to an incorrect value in release builds (the native workspace sets overflow-checks = false for release).

This is reachable from two places:

• Casting an integer type (Byte/Short/Integer/Long) to/from a negative-scale DecimalType, or casting a negative-scale DecimalType to TimestampType, both hit the same 10^|scale| computation in numeric.rs / cast_decimal_to_timestamp.
• Arithmetic on negative-scale decimals (Add/Subtract/Divide/IntegralDivide/Remainder), which scale-aligns operands the same way.

CometCast already special-cased negative-scale decimals for the Decimal -> String direction, but that guard was unreachable in practice because producing the negative-scale value in the first place would panic before a -> String cast could ever run.

Since allowNegativeScaleOfDecimal is a rarely-used legacy flag (default false), and the native kernels don't implement negative-scale-safe rescaling, the fix makes Comet detect these cases at planning time and fall back to Spark instead of attempting them natively.

What changes are included in this PR?

• CometCast.scala: added negativeScaleDecimalCastReason and marked as Unsupported:

  • int -> Decimal(neg) via canCastFromByte/Short/Int/Long
  • Decimal(neg) -> int and Decimal(neg) -> Timestamp via canCastFromDecimal (now takes the source DecimalType so it can check fromType.scale)

  All other cast directions to/from negative-scale decimals (e.g. String <-> Decimal(neg), Decimal(neg) -> Float/Double/String/Boolean/wider-Decimal) don't perform integer scale-alignment and are unaffected, they keep running natively.

• arithmetic.scala: added negScaleDecimalRejection(expr: BinaryArithmetic) in MathBase, returning Unsupported when the left operand, right operand, or result type is a negative-scale DecimalType. Wired into CometAdd, CometSubtract, CometDivide, CometIntegralDivide, CometRemainder.

  CometMultiply and UnaryMinus are intentionally left unguarded, neither scale-aligns, so they stay native

How are these changes tested?

Added tests to CometCastSuite and CometExpressionSuite
make core
./mvnw -pl spark test -Dsuites='org.apache.comet.CometCastSuite'
./mvnw -pl spark test -Dsuites='org.apache.comet.CometExpressionSuite'

Note: the TimestampType case is not called out in the linked issue's reproducer, it was found while auditing the same 10^|scale| code path during this fix and is included here since it shares the exact same root cause and fix.

[andygrove]

This may overlap with #4799

[0lai0]

Thanks @andygrove and @comphead, sorry for missing #4799! That would be great if it covers it.
I'd be glad to help review #4799.