fix: downcast to FixedSizeBinaryArray in CASE bytes lookup table

[jdymitarai]

Which issue does this PR close?

Closes #26012.

Rationale for this change

When evaluating CASE <expr> WHEN <literal> ... with two or more literal branches of byte-like types, DataFusion optimizes branch evaluation using LiteralLookupTable backed by BytesLikeIndexMap.

In try_get_bytes_iterator, the match arm for DataType::FixedSizeBinary(_) was accidentally calling array.as_binary::<i64>().into_iter(), which attempts to downcast to GenericBinaryArray<i64> (LargeBinaryArray). When presented with a FixedSizeBinaryArray, Arrow's downcast fails and panics:

thread 'main' panicked at arrow-array-60.0.0/src/cast.rs:938:29: binary array

Note that the dictionary arm in the same file (get_bytes_iterator_for_dictionary) already used .downcast_dict::<FixedSizeBinaryArray>(), confirming this was an oversight in the flat array branch.

What changes are included in this PR?

• Correct the downcast in try_get_bytes_iterator for DataType::FixedSizeBinary(_) to use array.as_fixed_size_binary().into_iter().
• Add unit tests in datafusion/physical-expr/src/expressions/case.rs covering both FixedSizeBinary and dictionary-encoded Dictionary(UInt8, FixedSizeBinary) CASE evaluation.
• Add regression tests in datafusion/sqllogictest/test_files/case.slt reproducing the issue query and verifying matching/non-matching branches.

Are these changes tested?

Yes:

• Added unit tests case_with_expr_fixed_size_binary and case_with_expr_fixed_size_binary_dictionary in datafusion-physical-expr.
• Added end-to-end regression tests in case.slt.
• Verified all new and existing tests pass cleanly with cargo test and sqllogictest.

Are there any user-facing changes?

No API changes. Fixes a panic when evaluating CASE expressions with FixedSizeBinary literal branches.