feat: expose scheduler fresh prefill counter

[FujitsuPolycom]

Summary\n- expose �llm:prefill_tokens_scheduled_total from scheduler-issued fresh prompt work\n- carry the count with each SchedulerOutput for async-safe accounting\n- record scheduler stats even when a chunked-prefill step has no output token yet\n- exclude prefix-cache and external-KV tokens\n\n## Validation\n- Python compilation passed for changed modules\n- scheduler assertions cover initial and chunked prefill scheduling\n\nThis is intended for live Prometheus consumers such as vLLM Observer. The metric measures fresh prompt work issued per scheduler step, not logger snapshots or end-to-end latency.

[github-actions]

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