invalid_key (HTTP 401) can be transient during key rotation or
temporary API issues — unlike rejected/exhausted which are stable
budget states. When cache shows all chain providers as invalid_key,
bypass cache and probe fresh so recovery is immediate instead of
waiting for the 20-minute TTL to expire.
Selects optimal Teams provider from chain based on real 7d utilization.
Uses cached monitor data (no extra API calls if fresh cache exists).
- Phase 1: first provider in chain with 7d util < SWITCH_THRESHOLD (default 75%)
- Phase 2: all over threshold → pick lowest 7d allowed provider
- Phase 3: all rejected → emergency=true, signals shelley-proxy needed
- Always fails safe: returns team-vigilio on any error
