fix: gate case conversion on standard per-expression allowIncompatible

[andygrove]

Which issue does this PR close?

Closes #4467.

Rationale for this change

CometCaseConversionBase (the parent of CometUpper and CometLower) gated its native opt-in on the bespoke spark.comet.caseConversion.enabled config, while its sibling CometInitCap already uses the standard per-expression allowIncompatible flow. This asymmetry was confusing: a user who set the standard spark.comet.expression.Upper.allowIncompatible=true (or Lower) saw no effect, because upper/lower were reading a different, bespoke flag.

Since #4721, the default path for upper/lower is already Spark-compatible via the JVM codegen dispatcher, with the native scalar function offered as a faster opt-in. The only remaining inconsistency was which config key drives that opt-in. This PR aligns upper/lower with initcap so both report support level and gate the opt-in through the same standard per-expression mechanism.

What changes are included in this PR?

• CometCaseConversionBase now gates on CometConf.isExprAllowIncompat(getExprConfigName(expr)) and surfaces the opt-in via getExprAllowIncompatConfigKey(...), matching CometInitCap. The nativeOptInConfigKeyOverride override is dropped, so upper/lower use spark.comet.expression.Upper.allowIncompatible and spark.comet.expression.Lower.allowIncompatible.
• Removed the now-redundant COMET_CASE_CONVERSION_ENABLED (spark.comet.caseConversion.enabled) config, whose original premise (disabling upper/lower by default) no longer holds now that the default path is Spark-compatible.
• Regenerated the Upper [COMET-INFO] opt-in hint in the affected tpcds plan-stability goldens (q24a, q24b, q24) to reference the new config key.
• Updated the predicate-functions expression audit note and the upper opt-in EXPLAIN test to reference the standard key.

How are these changes tested?

• Existing upper/lower SQL file tests (upper.sql, upper_enabled.sql, lower.sql, lower_enabled.sql). The _enabled fixtures were already written against the standard spark.comet.expression.{Upper,Lower}.allowIncompatible key and now actually exercise the native opt-in path.
• Updated CometExpressionSuite test asserting the EXPLAIN opt-in hint references spark.comet.expression.Upper.allowIncompatible.
• Plan-stability goldens are validated by the per-version plan-stability CI jobs.