Pretty-print builtin frozendict in assertion diffs

[HakeemTheEmperor]

Closes #15172

On Python 3.15, assertion diffs printed frozendict on a single line because
PrettyPrinter._dispatch had no entry for it, so it fell back to a plain repr.
This registers frozendict on 3.15+, reusing _pprint_ordered_dict since both
types repr as Name({...}). Subclasses use their own class name.

The registration and test are guarded by sys.version_info >= (3, 15), with
# noqa: F821 because ruff checks against Python 3.10, where frozendict
doesn't exist.

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