fix(providers): replay ambiguous resets on the OpenCode Go initial send

[agentHits]

Summary

Follow-up to #5067/#5158. The patient 429 fallback covers real upstream throttles, but a second 429 shape kept surfacing untouched: the Zen gateway stalls-then-drops inference sends, and the ambiguous pre-header reset becomes a synthetic non-replayable 429 (deliberately never replayed, never logged as anything but a 429). One such case (18s stall, single send, no recovery) aborted a user goal with exceeded retry limit.

This scopes a bounded reset replay to the OpenCode Go initial send on the Responses passthrough lane (replaySafe, still capped by the shared per-request send budget). Rationale for the scope: Zen/Go traffic is subscription inference-only, so a replay cannot duplicate anything but compute; every other destination keeps the fail-closed refusal, and recovery legs are untouched. A second, purely observational change: the log classifier now names a proxy refusal by its message (upstream_reset_replay_refused) instead of logging it as provider throttling (rate_limit_exceeded).

Verification

• bun test tests/usage/request-log.test.ts tests/responses/responses-passthrough-transient-policy.test.ts tests/lib/upstream-retry.test.ts tests/providers/upstream-transient-retry.test.ts tests/providers/rate-limit-retry.test.ts — 164 pass, 0 fail, including new cases for the refusal code and the single-legged replaySafe wiring.
• bun run typecheck — clean.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed (code comments carry the contract).
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults (replay is same-request, same-key, same body; OAuth/forward/local paths untouched).

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📥 Commits

Reviewing files that changed from the base of the PR and between b9d430b and 366480c.

📒 Files selected for processing (6)

• src/lib/errors.ts
• src/providers/key-failover.ts
• src/server/request-log.ts
• src/server/responses/passthrough-dispatch.ts
• tests/responses/responses-passthrough-transient-policy.test.ts
• tests/usage/request-log.test.ts

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Review readiness checklist

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