[Bug] repeat throws on negative count where Spark returns empty string

[andygrove]

Describe the bug

CometStringRepeat delegates to DataFusion repeat. DataFusion's repeat throws on negative n, while Spark's UTF8String.repeat returns the empty string for n <= 0. Comet currently reports Compatible for this expression (with a getCompatibleNotes caveat), so users with repeat(s, -1) get a runtime exception under Comet instead of the empty string Spark would produce.

Surfaced by the string-expressions audit in #4461.

Steps to reproduce

SELECT repeat('abc', -1);

Spark: returns ''.
Comet: throws ArrowError("Invalid argument error: repeat requires a non-negative number of repetitions") at execution.

Expected behavior

Either match Spark by returning '', or promote CometStringRepeat to Incompatible(Some(...)) so the path falls back unless explicitly enabled via spark.comet.expression.StringRepeat.allowIncompatible=true.

Additional context

• Comet serde: spark/src/main/scala/org/apache/comet/serde/strings.scala (CometStringRepeat)
• Spark reference: UTF8String.repeat(n) short-circuits for n <= 0
• The current getCompatibleNotes text mentions the divergence but the support level is still Compatible, so the path is taken silently.