[Bug] map_from_arrays / map_from_entries do not enforce null-key rejection or spark.sql.mapKeyDedupPolicy

### Describe the bug

`MapFromArrays` and `MapFromEntries` both route through Spark's `ArrayBasedMapBuilder`, which:

1. Throws `RuntimeException("Cannot use null as map key")` when a key is NULL.
2. Applies `spark.sql.mapKeyDedupPolicy` (`EXCEPTION` vs `LAST_WIN`) for duplicate keys.

Comet does not enforce either behavior:

- `CometMapFromArrays` (`spark/src/main/scala/org/apache/comet/serde/maps.scala`) only wraps the call in a CASE WHEN that handles whole-array NULLs. It does not detect a NULL element inside the keys array, and it does not implement either dedup policy.
- `CometMapFromEntries` (`maps.scala`) only gates on `BinaryType` keys / values. The null-key and duplicate-key cases are unmarked.

For datasets containing a NULL key or duplicate keys, Comet will silently produce a map where Spark would throw, or apply different dedup semantics.

### Steps to reproduce

Build a map via `map_from_arrays` or `map_from_entries` where the keys array contains a NULL element, or contains duplicate keys, and compare Comet against Spark with `spark.sql.mapKeyDedupPolicy` set to both `EXCEPTION` and `LAST_WIN`.

### Expected behavior

`CometMapFromArrays` and `CometMapFromEntries` should declare an `Incompatible(Some(...))` branch (or a tighter input check) covering null-key rejection and dedup-policy semantics, with matching entries in `getIncompatibleReasons()`, so the cases fall back to Spark rather than diverging silently.

### Additional context

Split out from #4505 (items 2 and 3), surfaced by the `audit-comet-expression` skill run in #4478. The two expressions share the `ArrayBasedMapBuilder` semantics so they are tracked together here. Distinct from #3327 (closed; native crash on whole-array NULL inputs).