fix: downcast to FixedSizeBinaryArray in CASE literal lookup table

[patrickswedish]

Which issue does this PR close?

Closes #26012.

Rationale for this change

In LiteralLookupTable, evaluation of CASE <expr> WHEN <literal> ... with byte-like literals delegates to BytesLikeIndexMap. In try_get_bytes_iterator, the DataType::FixedSizeBinary(_) match arm erroneously attempted to downcast the array via as_binary::<i64>() (inherited from DataType::LargeBinary), rather than handling FixedSizeBinaryArray.

When a query contained 2 or more FixedSizeBinary WHEN literals, LiteralLookupTable was constructed and panicked during evaluation when downcasting to GenericBinaryArray<i64>:

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

What changes are included in this PR?

1. Fix Downcast in Lookup Table: Updated DataType::FixedSizeBinary(_) arm in try_get_bytes_iterator (bytes_like_lookup_table.rs) to downcast via array.as_fixed_size_binary().iter(), matching the dictionary downcasting path and returning byte slices correctly.
2. Unit Test: Added test_case_when_literal_lookup_fixed_size_binary_to_int32 in datafusion/physical-expr/src/expressions/case.rs testing LiteralLookupTable evaluation with FixedSizeBinary literals with and without ELSE, with and without NULLs.
3. Sqllogictest: Added regression tests in datafusion/sqllogictest/test_files/case.slt verifying FixedSizeBinary CASE evaluation against branch 1, branch 2, fallback, NULL inputs, and multiple rows.

Are these changes tested?

Yes:

• Unit test in datafusion/physical-expr/src/expressions/case.rs (test_case_when_literal_lookup_fixed_size_binary_to_int32)
• Sqllogictest in datafusion/sqllogictest/test_files/case.slt
• Validated locally with cargo test and cargo clippy.

Are there any user-facing changes?

Bug fix: queries with CASE expressions containing multiple FixedSizeBinary literals now evaluate correctly instead of panicking.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.72%. Comparing base (db83fcc) to head (bc969b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26096      +/-   ##
==========================================
- Coverage   82.72%   82.72%   -0.01%     
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  Files        1147     1147              
  Lines      448179   448214      +35     
  Branches   448179   448214      +35     
==========================================
+ Hits       370754   370778      +24     
- Misses      54895    54903       +8     
- Partials    22530    22533       +3     

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