New HTTP handlers for Anthropic platform groups accepting OpenAI-format
endpoints:
- GatewayHandler.Responses: /v1/responses for non-OpenAI groups
- GatewayHandler.ChatCompletions: /v1/chat/completions for non-OpenAI groups
Both handlers include:
- Claude Code only restriction (403 reject when claude_code_only enabled,
since these endpoints are never Claude Code clients)
- Full auth → billing → user/account concurrency → failover loop
- Ops error/endpoint context propagation
- Async usage recording via worker pool
Error responses use each endpoint's native format (Responses API format
for /v1/responses, CC format for /v1/chat/completions).
