[SPARK-56153][SQL][TESTS][FOLLOWUP] Remove unnecessary asInstanceOf[classic.Dataset] casts in QueryTest

[zhengruifeng]

What changes were proposed in this pull request?

Remove three unnecessary asInstanceOf[classic.Dataset[_]] casts in QueryTest.scala (assertCached and assertEmptyMissingInput methods). The api-level Dataset already declares queryExecution (with @ClassicOnly annotation, which is purely documentation), so the casts are redundant.

Why are the changes needed?

The casts are unnecessary because:

1. The api-level Dataset already declares def queryExecution: QueryExecution — calling it directly works without casting to classic.Dataset.
2. The same file already calls ds.queryExecution directly without a cast on line 126 (getResult method), confirming this is safe.
3. The @ClassicOnly annotation is a runtime documentation annotation with no compile-time effect.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing tests (unable to compile locally due to missing lz4-java:1.11.0 dependency in environment).

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

Co-authored-by: Claude code (Opus 4.6)

[HyukjinKwon]

LGTM if it works

[zhengruifeng]

merged to master