## Description
Adds automatic key failover for centralized OpenAI provider, covering both chat completions and responses APIs. Same shape as the Anthropic PR: each upstream call walks the configured key pool, keys are marked **temporary** on 429 (with cooldown from `Retry-After`) and **permanent** on 401/403. Each agentic-loop iteration gets its own fresh walker so a tool-call continuation can fail over independently of the initial request.
BYOK is unchanged: BYOK requests run as a single attempt with no failover.
## Changes
- `config.OpenAI` carries a `KeyPool`. `Key` remains for BYOK Authorization Bearer set per interception.
- Chat completions blocking interceptor: walks the pool via `newChatCompletionWithKeyFailover`, marks keys on key-specific failures, returns on first success or non-failover error.
- Chat completions streaming interceptor: per-iteration walker. Pre-stream failures fail over to the next key; mid-stream errors are relayed as SSE events.
- Responses blocking interceptor: extracts `newResponseWithKeyFailover` parallel to chatcompletions.
- Responses streaming interceptor: per-iteration walker, retains the existing buffer-then-forward design.
## Related Issues
Related to: https://github.com/coder/internal/issues/1446
Related to: https://linear.app/codercom/issue/AIGOV-197/aibridge-automatic-key-failover-for-bridged-and-passthrough-routes
## Follow-up PRs
- Bedrock multi-key support.
- Refactor provider vs interceptor config separation.
- Record the actually-used key in the interception credential hint after failover.
> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by @ssncferreira
