Fix uniq comparisons across hashability paths

[Sanjays2402]

uniq() could report equal values as unique when one uses the structural-hash path and the other falls back to pairwise comparison (for example, set and frozenset). Compare fallback values across both paths so the result no longer depends on input order.

[Julian]

Hi there -- such values aren't possible to encounter in JSON as far as I can tell from your patch. Is there some actual user-facing bug you see via this path (i.e. a schema)? If so we'd at least want to test via that.

[Sanjays2402]

@Julian You're right. I traced the complete _uniq_key() path: every strict JSON type is structurally supported recursively (objects have string keys; arrays recurse; scalars are hashable). _UNSUPPORTED only covers values outside the JSON data model, such as set, custom mappings with unhashable keys, or NaN. The current set/frozenset regression therefore cannot be reached from a schema-valid JSON instance.

I could not produce a schema-level user-facing case, so I'm withdrawing this PR rather than expanding uniq() for non-JSON Python values.