fix: align dictionary coercion across typed signatures

[lyne7-sc]

Which issue does this PR close?

• Follow-up to perf: preserve dictionary encoding for lower/upper to avoid materializing low-cardinality columns #22905.

Rationale for this change

#22905 introduced explicit dictionary encoding preservation for coercible function signatures, but dictionary inputs were still handled differently across TypeSignatureClass variants:

Signature category	Before	After
Native(...)	Materialized by default; preserved when explicitly requested	Same default/opt-in contract
Typed non-Native (e.g. Integer, Numeric, Binary)	Retained the physical dictionary type by default	Materialized by default; preserved when explicitly requested
Any	Passed through the original physical input type	Unchanged

This PR makes the encoding preservation contract consistent across all typed signature classes: coercion operates on the dictionary value type, and the dictionary encoding is restored only when EncodingPreservation::dictionary() is enabled.

An audit of the affected built-ins found two functions, Spark hex and bitmap_count, that intentionally handle dictionary inputs; both now opt in explicitly. Other affected functions generally expect materialized value arrays, so the new default also avoids cases where signature matching accepted a dictionary but the function implementation rejected it at execution time. Functions that continue to materialize dictionary inputs do not gain dictionary-aware execution efficiency yet, but they can opt in later if they add support for encoded inputs.

What changes are included in this PR?

• Align dictionary coercion across typed signature classes and preserve dictionary encoding when explicitly requested.
• Explicitly enable dictionary preservation for Spark bitmap_count and the binary variant of Spark hex.
• Document the behavior change and migration guidance in the DataFusion 55.0.0 upgrade guide.

Are these changes tested?

• Unit tests cover materialization and preservation for Native, non-Native, and Any inputs.
• SLTs cover to_hex materialization and verify that bitmap_count preserves its dictionary input without an additional cast to Binary.
• Existing Spark hex dictionary tests cover its opt-in preservation behavior.

Are there any user-facing changes?

This is a behavioral API change for UDFs using typed non-Native classes such as Integer or Binary. UDFs relying on implicit dictionary preservation must now enable EncodingPreservation::dictionary() explicitly.

TypeSignatureClass::Any is unaffected. The upgrade guide has been updated, and this PR should carry the api change label.