Map lookups with float, collated or complex keys fall back to Spark (`map_col[key]`, `element_at`)

[andygrove]

Part of #5572.

MapKeySupport.keySupport (spark/src/main/scala/org/apache/comet/serde/maps.scala:68) returns Unsupported for three classes of map key:

• floating point anywhere in the key type, because Spark normalizes -0.0 to +0.0 and canonicalizes NaN in ArrayBasedMapBuilder while native map_extract compares raw Arrow values
• a non-default collation, because native compares as UTF8_BINARY
• any complex key type, because map_extract's coercion casts the lookup key to the map's exact Arrow key type

Two serdes consume it, and neither mixes in CodegenDispatchFallback:

• CometMapExtract (GetMapValue, i.e. map_col[key]) — serde/maps.scala:117, gate at :120
• CometElementAt on map input — serde/arrays.scala:614

So map_col[key] and element_at(map_col, key) fail the whole projection back to Spark whenever the map has double, collated-string or struct keys.

The analysis behind each decline is right — these really are cases the native lookup gets wrong — which is exactly why the dispatcher is the correct answer rather than a native fix. Spark's own GetMapValue.doGenCode and ElementAt.doGenCode give the normalization and interpreted-ordering equality for free, and MapType is fully supported by CometBatchKernelCodegen.isSupportedDataType (recursively, as long as the leaves are).

Fix: mix CodegenDispatchFallback into both serdes.

Note that CometElementAt's other Unsupported arm, "Input must be an array or map", is unreachable — Spark's own type checking rejects that before Comet sees it — so it needs no handling.