Adds an opt-in tier-based fallback scheduling path for Anthropic accounts:
- accountTierLevel(): derives tier from account type without DB migration
(tier-0=OAuth/SetupToken, tier-1=APIKey, tier-2=Bedrock)
- enableTierFallbackChain(): new config flag
gateway.scheduling.enable_tier_fallback_chain (default false)
- selectAccountWithTierFallback(): loads all Anthropic accounts, groups by
tier, honors sticky sessions, applies all existing schedulability guards,
then tries tiers 0→1→2 in order via tryAcquireByLegacyOrder
- Wired into SelectAccountForModelWithExclusions: Anthropic platform +
tier fallback enabled → calls new path instead of mixed scheduling
- Fix pre-existing unit-test build break: NewGatewayService now requires
*RPMTokenBucketService (added in Task #5); add missing nil param
- 7 tests: tier mapping, config toggle, subscription preference,
APIKey fallback, exclusion handling, empty-pool error, Bedrock last resort
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