Use arrow dictionary casts instead of hand-rolled dictionary handling in cast paths

[andygrove]

What is the problem the feature request solves?

Two dictionary-handling paths hand-roll what arrow's cast kernel supports natively:

• native/spark-expr/src/conversion_funcs/cast.rs dict_from_values (explicitly "copied from datafusion common scalar/mod.rs") manufactures a degenerate dictionary with one key per row and no deduplication, then the cast recurses into it.
• native/core/src/parquet/parquet_support.rs parquet_convert_array has a branch that manually unpacks Dictionary(Int32, Utf8/LargeUtf8) via recursive value conversion plus take, or rebuilds a dictionary by hand.

Describe the potential solution

• In cast.rs, invert the order: Spark-cast the plain array to the dictionary's value type first, then arrow::compute::cast to Dictionary(key, value). Arrow's cast_to_dictionary handles key creation and dedups values via dictionary builders. The Spark-owned semantics stay in the value cast.
• In parquet_support.rs, the dictionary branch can likely be deleted: since the value type is restricted to Utf8/LargeUtf8, the recursion can only hit identity or the generic can_cast_types fallthrough, and arrow's cast_with_options natively supports both Dictionary -> T (unpack via take) and Dictionary -> Dictionary (cast values).

Additional context

• Output representation differs in cast.rs (deduplicated dictionary instead of one-entry-per-row), which is logically the same array; downstream consumers treat dictionaries generically.
• Before deleting the parquet branch, verify can_cast_types(Dictionary(..), to) covers the same combinations, since unsupported combos currently fall through to returning the original array.

Found during an audit of native code that replicates existing arrow-rs kernels.

[peterxcli]

take