[SPARK-53535][SQL][FOLLOWUP] Fix findCheapestGroupField returning invalid Map

[ZiyaZa]

What changes were proposed in this pull request?

This PR fixes a bug from #52557, where we are reading an additional field if all the requested fields of a struct are missing from the Parquet file. We used to always pick the cheapest leaf column of the struct. However, if this leaf was inside a Map column, then we'd generate an invalid Map type like the following:

optional group _1 (MAP) {
repeated group key_value {
required boolean key;
}
}

Since there is no value field in this group, we'd fail later when trying to convert this Parquet type to a Spark type. This PR changes the additional field selection logic to enforce selecting a field from both the key and the value of the map, which can now give us a type like following:

optional group _1 (MAP) {
repeated group key_value {
required boolean key;
optional group value {
optional int32 _2;
}
}
}

Why are the changes needed?

To fix a critical bug where we would throw an exception when reading a Parquet file.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New unit tests.

Was this patch authored or co-authored using generative AI tooling?

No.

[cloud-fan]

thanks, merging to master!