Spark: Add regression test for MERGE struct nullification

[yadavay-amzn]

Closes #16246.

Summary

Adds a regression test verifying that MERGE INTO preserves non-nullable children of a nullable struct column when updating other columns.

Context

Issue #16246 reported that MERGE INTO silently nullified non-nullable struct children on Iceberg 1.5.2 / Spark 3.3. The bug is already fixed on main (likely resolved by schema handling improvements in later versions). This test prevents reintroduction.

Test

testMergePreservesNullableStructWithRequiredChildren:

1. Creates a table with schema id INT, status STRING, info STRUCT<type:STRING NOT NULL, attr:STRING NOT NULL>
2. Inserts a row with non-null struct data
3. Performs MERGE INTO updating only the status column
4. Asserts struct children remain non-null