Serverless 2.0: Three Ways to Run Inference, One API

Introduced Serverless 2.0 with three distinct serving paths: Standard (default, cost-efficient, sheds first under load), Priority (stronger admission during congestion, sheds last), and Fast (high-throughput serving path for supported models, ~2x Standard throughput). Differentiated HTTP 429 (rate limit) from 503 Service Overloaded (fleet saturation) and 503 Service Unavailable (platform issue). Priority is activated via `service_tier: "priority"` and Fast via specific model router IDs (e.g., `accounts/fireworks/routers/kimi-k2p6-turbo`).