fix(codex): same-request multi-account failover on quota 429 (#584)

[Wibias]

Summary

• Fix Multi-account failover issue: Session freezes on depleted primary account despite active secondary Pro account #584: when the active Codex pool account returns a pre-stream 429/402, retry once on an eligible alternate account in the same request (mirrors the existing allow-listed model-400 pool retry and API-key 429 rotation).
• Sticky threads whose bound account is already over �utoSwitchThreshold rebind immediately instead of waiting up to 60s.
• Docs note + regression tests for HTTP and streamed pre-stream 429 failover, single-account fail-closed, and immediate affinity rebind.

Why

Routing already cooled the depleted account and switched �ctiveCodexAccountId after a 429, but the failing turn was still returned to Codex CLI — which shows the limit/overuse UI and stalls until the user manually retries. Same-request failover makes the secondary Pro account transparent.

Test plan

• �un run typecheck
• �un test tests/server-auth.test.ts -t "Activation|pool retry|Multi-account failover issue: Session freezes on depleted primary account despite active secondary Pro account #584"
• �un test tests/codex-routing.test.ts
• �un run privacy:scan
• CI green on Linux/Windows/macOS
• Manual: primary Plus exhausted + secondary Pro with quota → multi-step Codex CLI session continues without limit stall

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📥 Commits

Reviewing files that changed from the base of the PR and between 94a83bc and e589087.

📒 Files selected for processing (5)

• docs-site/src/content/docs/reference/configuration.md
• src/codex/routing.ts
• src/server/responses/core.ts
• tests/codex-routing.test.ts
• tests/server-auth.test.ts

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