Core: Order Variant metadata dictionary and object fields by UTF-8 byte order

[nssalian]

Rationale for this change

Variant metadata stores field names in a dictionary. When its sorted_strings flag is set, VariantEncoding.md requires those names to be in UTF-8 byte order.

Iceberg sorts and looks them up with Java's String.compareTo, which is UTF-16 order. For ASCII and most characters the two orders are identical, but they differ for characters above U+FFFF. So Iceberg can flag a dictionary as sorted when it is not actually UTF-8-ordered. Iceberg reads its own files fine because it sorts and searches the same way, but a spec-conforming reader (iceberg-go, iceberg-rust) searches by UTF-8 and would fail to find those fields.

Changes

Sort and look up Variant field names with the existing UTF-8 comparator Comparators.charSequences() at every site:

• Variants.metadata - sets the dictionary sorted_strings flag
• VariantUtil.find - reader lookup over field names
• ShreddedObject - shredded object field order
• RecordConverter (kafka-connect) and ParquetMetrics - sort field names before building the dictionary

No on-disk format or spec change - this conforms Iceberg to the existing spec. Files with only ASCII or common field names are unaffected; only names above U+FFFF change ordering.

Testing

TestVariantMetadataFieldOrdering and a new TestShreddedObject case check that a dictionary is flagged sorted only when it is actually UTF-8-ordered, and that fields with characters above U+FFFF are found on read. The tests fail on the old String.compareTo code and pass on the fix (verified by reverting the change and watching them go red).