Remove stale dev/diffs test exclusions whose tracking issues are closed #5569

Three exclusions in dev/diffs/*.diff cite Comet issues that have since been fixed, so they now suppress Spark SQL test coverage for no reason. Each is present in more than one version diff, so they should be removed across all affected diffs in one pass rather than in whichever version PR happens to touch them next: new version diffs are seeded from the previous version's, so a partial fix diverges the diffs and the next reseed reintroduces the stale entry.

Surfaced in review of #4950 by @sunchao.

1. Variant shredding suites

VariantShreddingSuite and ParquetVariantShreddingSuite are given an IgnoreCometSuite mixin, which skips all 15 tests when ENABLE_COMET=true.

• Cited issue: [native_iceberg_compat] VariantShreddingSuite test failures with Spark 4.0.0 #2209, closed by merged PR fix: fall back for shredded Variant scans on Spark 4.0 #4084.
• fix: fall back for shredded Variant scans on Spark 4.0 #4084 added the shredded-Variant scan fallback and already removed these same exclusions from dev/diffs/4.0.4.diff (it has zero occurrences of VariantShreddingSuite); 4.1.3.diff was never cleaned up.
• Affected: dev/diffs/4.1.3.diff, dev/diffs/4.2.0.diff.
• Removing the mixins should also drop the now-unused IgnoreCometSuite import in each file.

This is the highest-value one: it drops regression coverage for the silent-null corruption that #4084 fixed.

2. having-and-order-by-recursive-type-name-resolution.sql

The input file carries a --SET spark.comet.enabled = false header, which runs all 45 queries without Comet.

• Cited issue: Comet native sort lacks row-format support for Struct(Map(...)) sort keys #4123, closed by merged PR fix: prevent native sort crash for Struct(Map(...)) keys #4157, which added recursive fallback for map-containing aggregate keys plus a regression test for the exact Struct(Map(...)) HAVING / ORDER BY shape.
• Affected: dev/diffs/4.1.3.diff, dev/diffs/4.2.0.diff.

3. Parquet V2 writer matrix in ParquetTypeWideningSuite

withAllParquetWriters { ... } is commented out, leaving all 82 generated widening cases on Parquet's default V1 writer.

• Cited issue: Support DELTA_BINARY_PACKED and DELTA_BYTE_ARRAY #574, closed: the active native_datafusion and native_iceberg_compat scans support DELTA_BINARY_PACKED, DELTA_BYTE_ARRAY and DELTA_LENGTH_BYTE_ARRAY.
• The related SPARK-40128 read DELTA_LENGTH_BYTE_ARRAY encoded strings test in ParquetIOSuite is still tagged IgnoreComet("Comet doesn't support DELTA encoding yet") and should be re-enabled with it.
• Affected: dev/diffs/4.0.4.diff, dev/diffs/4.1.3.diff, dev/diffs/4.2.0.diff (byte-identical in all three).
• If the legacy native_comet scan still cannot read these encodings, scope the exclusion to that scan implementation rather than removing the whole matrix.

Suggested approach

One PR per item, each removing the exclusion from every diff that carries it, verified by the corresponding Spark SQL Tests CI job for each affected version. Where a specific case still fails, replace the blanket exclusion with a case-scoped IgnoreComet linked to a current issue, per the rule in docs/source/contributor-guide/adding_a_new_spark_version.md ("do not disable whole suites").