fix(zen): read nested usage in non-stream responses

[kitlangton]

Summary

• extract non-stream usage from each provider's actual response shape instead of assuming every provider returns top-level usage
• fix the OpenAI Responses path where usage can be nested under response.usage, which was causing Zen to throw before tracking or returning the response
• add focused provider usage extraction tests and a debug log for responses that still arrive without any usage payload

Verification

• bun test test/provider-usage.test.ts test/rateLimiter.test.ts
• bun run --cwd packages/console/app typecheck