Deliberately opt eligible Incompatible expressions into codegen dispatch with test coverage

### Background

PR #4538 introduced an opt-in `CodegenDispatchFallback` marker trait. When an expression reports `Incompatible` and the user has not enabled `allowIncompatible`, mixing in the trait routes it through the JVM codegen dispatcher (Spark's own `doGenCode` inside the Comet pipeline) so the projection stays native while matching Spark exactly, instead of falling the whole projection back to Spark.

Before that, enrollment was opt-out, which silently routed a number of native `Incompatible` expressions through the dispatcher without deliberate review or test coverage. Those were reverted to Spark fallback (matching `main`) in PR #4538. This issue tracks deliberately opting them back in, one at a time, each with explicit test coverage proving the dispatched path matches Spark.

### Candidate expressions

These report `Incompatible` for some inputs, have a real Spark `doGenCode` (so the dispatcher can accept them), and currently fall back to Spark:

- [ ] `concat` (`CometConcat`) - non-UTF8_BINARY collation
- [ ] `array_intersect` (`CometArrayIntersect`)
- [ ] `array_except` (`CometArrayExcept`)
- [ ] `array_join` (`CometArrayJoin`)
- [ ] `from_utc_timestamp` (`CometFromUTCTimestamp`)
- [ ] `to_utc_timestamp` (`CometToUTCTimestamp`)
- [ ] `convert_timezone` (`CometConvertTimezone`)
- [ ] `sort_array` (`CometSortArray`) - verify dispatcher eligibility first; this one may be `CodegenFallback` in Spark, in which case the dispatcher refuses it and it should stay on Spark fallback

### What to do per expression

1. Confirm the dispatcher accepts it (not `CodegenFallback`, supported input/output types per `CometBatchKernelCodegen.canHandle`). If refused, leave it as Spark fallback and note it.
2. Mix `CodegenDispatchFallback` into the serde that is **registered** in `QueryPlanSerde` (not a delegate; see how `Reverse` registers `CometReverse`, which delegates to `CometArrayReverse`).
3. Add tests asserting native execution that matches Spark for the `Incompatible` inputs (`checkSparkAnswerAndOperator` / `query` in SQL file tests, replacing the `expect_fallback` assertion).
4. For the timezone-sensitive expressions, test across multiple session time zones, since dispatch correctness depends on the resolved `timeZoneId` surviving closure serialization.

### Notes

The expressions that have no native implementation and extend `CometCodegenDispatch` (`hypot`, `bround`, `sequence`, `elt`, ...) are out of scope: they report `Compatible` and already dispatch.