fix: support unhex on dictionary strings

[yuboxx]

What changes were proposed in this pull request?

This PR adds dictionary string support to the native Spark-compatible unhex expression. Dictionary arrays backed by Utf8 or LargeUtf8 values are unpacked and then handled by the existing string implementation.

It also expands coverage for unhex:

• Rust unit coverage for dictionary arrays with valid hex, invalid hex, null keys, and null dictionary values
• Spark expression coverage with dictionary enabled and disabled
• Scalar literal coverage with constant folding disabled
• SQL-file coverage under parquet.enable.dictionary=false,true

Closes #477.

How was this patch tested?

• cargo test -p datafusion-comet-spark-expr math_funcs::unhex::test
• ./mvnw -pl spark -Dsuites=org.apache.comet.CometExpressionSuite -Dtest=none -Pspark-4.0 -Pscala-2.13 test

[andygrove]

Thanks for the contribution @yuboxx. Could you tell me more about the motivation for this PR? The changes looks reasonable, but do not address any bugs as far as I can tell. Dictionary-encoded arrays are unpacked in parquet_convert_array before any expressions can be evaluated.