Test Account Pool family failover after inline retry

[ymichael]

Human comments

What was wrong

PR #3053 correctly switched accounts when the first upstream response declared a model-family limit, but its short same-account retry used a separate response path. If the first response was an ordinary short 429 and the retry then declared a family limit, that second path returned the 429 without trying another eligible account. PR #3117 later consolidated attempts into one loop and incidentally fixed the root cause on current main, but no regression test covered the original two-stage response sequence.

What changed

Added a shipped Account Pooler plugin test using the Plugin SDK fake-host harness. It drives the exact sequence sk-one ordinary short 429, sk-one family-limit 429, then sk-two success, and asserts the client receives HTTP 200. There is no production, wire-contract, CLI, guide, documentation, credential, quota, or unrelated-provider change.

How you verified

• Red on the original landing: HTTP 429 with attempts sk-one, sk-one; sk-two was not tried.
• Green on current main: the focused harness test passed with sk-one, sk-one, sk-two and HTTP 200.
• pnpm exec turbo run test typecheck build --filter=bb-plugin-account-pool --concurrency=1 --continue --force — 6 tasks passed; 10 test files and 262 tests passed; typecheck passed.
• pnpm exec oxfmt --check plugins/account-pool/src/server.test.ts and git diff --check origin/main...HEAD passed.

Part of #1552

AGENT GENERATED