Add preferred prefill hint routing

[ztorlakTT]

• Add an optional preferred_prefill_id hint that Dynamo/raw requests can pass through decode to PrefillGateway.
• Make PrefillGateway honor the preferred prefill only when it is healthy, accepting tasks, and under capacity; otherwise it falls back to prefix/load/round-robin routing.
• Add selector and dispatcher coverage for preferred-prefill routing and fallback behavior.