Avoid retaining nameless collectors after repeated registration

[zhechen-cs]

With support_collectors_without_names=True, registering the same collector twice adds two entries to _collectors_without_names, while the main registry stores it only once. After unregister(), normal collection is empty but restricted_registry() still calls the collector and exposes its stale metrics.

Keep the auxiliary list unique on registration. The regression covers repeated registration, normal and restricted collection, unregistering, and registering the collector again. This covers a remaining case after #1191, which fixed unregistering a collector registered once.

Validation: the regression fails on the unmodified base. After the fix, 429 tests pass and 14 are skipped; the configured flake8, isort and mypy checks pass. Local HTTP tests used NO_PROXY=localhost,127.0.0.1,::1 to bypass the local proxy.

@csmarchbanks