[SPARK-54975][SQL][TESTS] Refactor Merge Into Schema Evolution Tests

[szehon-ho]

What changes were proposed in this pull request?

The MergeIntoTableSuiteBase class had become unwieldy (7200+ lines), of which around 5000 lines were schema evolution tests

1. Move the many Merge Into Schema Evolution tests into their own file
2. Make helper methods testEvolution , etc (this is inspired by Delta Lake, see https://github.com/delta-io/delta/blob/master/spark/src/test/scala/org/apache/spark/sql/delta/MergeIntoSchemaEvolutionSuite.scala ). These will run the tests a. with schema evolution, 2. without schema evolution, and 3. using coerceNestedTypes on and off if applicable
3. Migrate all ~70 tests to use that framework
4. Add even more dimensions (group-based vs delta-based MERGE, Scala API, SQL)

Why are the changes needed?

1. This cuts down the tests from 5000 lines to less than 3000 lines.
2. Inspired by Delta Lake's test approach, this makes it a bit easier to see what is being tested by focusing on the differences in data/schema and expectations.

Does this PR introduce any user-facing change?

No, only test

How was this patch tested?

Run existing test

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

A bit using Claude 4.5 opus, but mostly hand tuning

[github-actions]

JIRA Issue Information

=== Test SPARK-54975 ===
Summary: Refactor Merge Into Schema Evolution Tests
Assignee: None
Status: Open
Affected: ["4.1.0"]

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[szehon-ho]

@cloud-fan can you help take a look? thanks

[cloud-fan]

is this for delta-based merge into only? because group-based test suites do not extend it.