`_entry_identity` compared the fallback entry's provider id as a raw,
lowercased string. A provider reachable under two ids — the canonical
models.dev id (`opencode`) and a legacy rename alias (`opencode-zen`) —
was therefore kept as two separate slots in the fallback chain. That shifts
the chain by one slot and lets an alias slip past the same-backend skip,
so a turn can hit the alias slot, recover from a re-seeded rate-limit pool
against it, and exhaust retries without ever reaching the real paid
fallback.
Normalize the provider id (lazy `normalize_provider` import to avoid
cycles) before computing the entry identity, so aliases collapse onto the
canonical id for dedup. The returned chain still carries each entry's
original provider id — only the identity used for dedup is normalized.
Fixes#85235
2fba6e31bd