length / bit_length / octet_length fall back to Spark on binary input #5584

Part of #5572.

length, bit_length and octet_length all decline BinaryType input, and each falls the whole projection back to Spark:

case _: BinaryType => Unsupported(Some("Length on BinaryType is not supported"))

spark/src/main/scala/org/apache/comet/serde/strings.scala:89 (CometLength), :98 (CometBitLength), :107 (CometOctetLength)

All three are trivial in Spark's generated code — length(binary) is .numBytes(), and the other two are arithmetic on it. BinaryType is in CometBatchKernelCodegen.isSupportedDataType, so the dispatcher handles them.

Fix: mix CodegenDispatchFallback into all three. They are near-identical serdes, so this is one small change.

The generated docs list all three as "✅ Native" with no note today, so they should pick up the Hybrid marker once the mixin is added.