test: cover struct data columns in native shuffle

[viirya]

Which issue does this PR close?

Closes #5563.

Rationale for this change

CometShuffleExchangeExec.supportedSerializableDataType allows struct data
columns in native shuffle, but CometNativeShuffleSuite never exercised one --
the only nested coverage was array<array<int>> and map<int, null>, and the
word struct did not appear in the suite at all. CometColumnarShuffleSuite
has extensive nested coverage, so the gap was specific to native shuffle.

Nested types in shuffle have been a real source of bugs (e.g. cd4d0e2, #5137,
on nested field nullability in ShuffleScanExec), so a behavior the type gate
explicitly claims to support deserves an end-to-end test rather than relying on
it not regressing.

What changes are included in this PR?

Test-only. Four tests added to CometNativeShuffleSuite:

• native shuffle on struct data column -- a plain struct<int, string>
• native shuffle on struct data column including nulls -- a null struct and a
  null field inside a struct
• native shuffle on deeply nested data columns -- struct<array<int>, string>
  and array<struct<int, string>>, covering the recursive branches of the gate
• native shuffle on struct data column with map field

All of them partition on a primitive column and carry the struct as a data
column, because supportedHashPartitioningDataType rejects nested types as a
hash key -- a struct can only reach native shuffle as data. The first test
carries a comment explaining this, so the arrangement is not mistaken for an
oversight.

No product code is changed.

How are these changes tested?

This PR is the tests. CometNativeShuffleSuite passes locally: 40 tests,
40 succeeded, 0 failed (36 pre-existing + 4 new).

I also mutation-tested them to confirm they are not vacuous: temporarily
changing the native gate's case StructType(...) to false makes exactly these
4 tests fail while the other 36 keep passing, so they really do assert that
struct data columns go through native shuffle, rather than passing because the
plan silently fell back to Spark.

[viirya]

Thanks @sunchao