Add pricing extraction (rate_multiplier, rate_unit, prompt/completion tokens) and auto-configure rate limits on 429

- Parse rate_multiplier and rate_unit from nexlab API as pricing
- Parse promptTokenPrice and completionTokenPrice from AWS Q API
- Extract pricing from OpenRouter-style API responses for OpenAI provider
- Add _auto_configure_rate_limits to extract X-RateLimit-* headers
- Update parse_429_response to capture rate limit headers