Add SerializationFeature.ORDER_SET_ENTRIES_BY_ELEMENTS for #3166

[dlwldnjs1009]

Summary

This adds a Phase 1 implementation for #3166 by introducing global Set ordering during serialization.

• add SerializationFeature.ORDER_SET_ENTRIES_BY_ELEMENTS (disabled by default)
• add SerializationFeature.FAIL_ON_ORDER_SET_BY_INCOMPARABLE_ELEMENT (enabled by default)
• add ordering hooks to both CollectionSerializer and StringCollectionSerializer
• add behavioral regression tests for ordered, null, incomparable, and typed paths

Implementation notes

Set<String> takes the specialized StringCollectionSerializer path, while other Set types use CollectionSerializer, so both serializers need the ordering hook.

The hook is placed in serializeContents(), which also covers the serializeWithType() path because AsArraySerializerBase delegates there directly.

SortedSet and EnumSet are left as-is.

For incomparable elements, the implementation attempts sorting and catches ClassCastException. FAIL_ON_ORDER_SET_BY_INCOMPARABLE_ELEMENT controls whether serialization should fail or simply skip sorting.

Tests

Adds CollectionSerializationOrderTest covering:

• feature disabled / enabled behavior
• integer and string Set ordering
• SortedSet and EnumSet
• null handling
• incomparable elements (fail and skip)
• polymorphic serialization
• custom content serializer fallback

Out of scope

Per-property annotation support (JsonFormat.Feature counterpart) is intentionally deferred to a follow-up change.

[cowtowncoder]

We are trying to stop adding new features in 2.x and mostly just apply fixes so I think this should go against 3.x.

[dlwldnjs1009]

Closing this in favor of a new PR against 3.x, per your suggestion. The implementation has been adapted to the 3.x APIs. Thank you for the guidance.