Which issue does this PR close?

• works towards Epic: Cost-Based Dictionary Encoding datafusion#24111
• for Add physical plan optimization to decide dictionary encoding datafusion#24117 we need to reduce the cost of selecting a key type that is too small for a given cardinality. this means making it as cheap as possible to bump the key type of a dictionary array.

Rationale for this change

Dictionary(K1, V) → Dictionary(K2, V) casts (changing only the key width) currently take the general path in dictionary_to_dictionary_cast, which does three things that are unnecessary when the value type is unchanged:

1. Calls cast_with_options on the values array — dispatch overhead even for identity casts, real work for near-identity casts.
2. Rebuilds the dictionary's ArrayData graph via into_builder().child_data(...).build_unchecked().
3. Wraps the result in DictionaryArray::<K2>::from(data), which runs an O(N_rows) bounds-check pass over every key.

If the source dict was valid and the key cast didn't overflow, the result is valid by construction

What changes are included in this PR?

A fast path in dictionary_to_dictionary_cast: when the source value type equals the target value type (and the source isn't a nested dict), delegate to cast_keys_only, which casts the keys array and reuses the values Arc via DictionaryArray::new_unchecked. Narrowing overflow remains a hard error regardless of cast_options.safe, same contract as the general path.

Are these changes tested?

Are there any significant user-facing changes?

no