[codex] add FIFO account wait queue

[xtyooo]

What changed

• Add an optional Redis-backed FIFO wait queue for account slots
• Only the queue head can acquire the next account slot
• Encode wait deadlines in tickets and remove expired or cancelled heads
• Keep the legacy counter and polling path for non-queue cache implementations
• Use the configured WaitPlan max waiting and timeout across Claude, Gemini, and OpenAI account paths
• Include integration coverage for FIFO ordering, capacity limits, and cancelled waiters

Why

The previous account wait path only counted waiters and let every request compete to acquire a slot. That allowed later requests to overtake earlier requests and caused avoidable slot-acquisition pressure during bursts.

This first slice makes admission order deterministic and bounds stale waiters. Waiters still use the existing exponential-backoff polling loop, but only the FIFO head performs the account-slot acquisition attempt; event-driven wakeups can be added in a follow-up without changing the queue contract.

Validation

• go test ./internal/handler ./internal/service ./internal/repository -count=1
• go test -tags integration ./internal/repository -run TestConcurrencyCacheSuite/TestAccountWaitQueue -count=1
• git diff --check

The queue is optional so existing cache implementations and test doubles retain the legacy behavior.

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