[regression] simplify iceberg table cache schema-change checks

[xylaaaaa]

Summary

• simplify schema-change section in test_iceberg_table_cache
• keep ADD COLUMN, RENAME COLUMN, ALTER COLUMN TYPE
• remove DROP COLUMN subcase to reduce heavy external Spark calls

Why

• keep key schema-cache coverage (including column-count increase path)
• reduce externregression runtime with a moderate cut instead of removing all column add/drop checks

Impact

• spark_iceberg calls reduced from 48 to about 44 in this case

Test

• not run in local environment (pipeline validation expected)

[Thearas]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[Copilot]

Pull request overview

Simplifies the Iceberg external table cache regression test by reducing the number of schema-evolution scenarios executed via Spark, aiming to keep representative cache-behavior coverage while lowering external-call cost in externregression.

Changes:

• Removed schema-change subcases for external ADD COLUMN and DROP COLUMN.
• Kept schema-change coverage for external RENAME COLUMN and ALTER COLUMN TYPE with cache vs no-cache verification.

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[Copilot]

In the ALTER COLUMN TYPE test, the cached-schema assertion uses contains("int"), but the expected new type bigint also contains the substring int. This can make the test pass even if the cache is already refreshed (or not caching). Use an exact comparison (e.g., equalsIgnoreCase("int")) or explicitly assert that the type is not bigint for the cached/no-refresh case (and similarly tighten the initial schema check if you want it to fail fast).

[xylaaaaa]

run buildall