[Feature]: General cross-instance KV-cache transfer/staging (kv_transfer)

🚀 The feature, motivation and pitch

Disaggregated serving already moves a request's KV from a context server to a generation server on
demand. We'd like to expose that same transceiver path as a standalone, proactive operation: tell
instance B to pull a given prompt's KV from instance A and commit it to B's reuse cache, without
generating.

Motivation. Today the only ways to get a prompt's KV onto a specific instance are to actually run
a request there (cold prefill) or to rely on on-demand disagg at request time. A proactive transfer
enables, all on the existing transceiver:

• Cache migration / rebalancing — move a hot prompt's KV off an overloaded instance onto an idle
  one, instead of recomputing it there.
• Replication — keep a copy of a prompt's KV on more than one instance.

Proposed surface (additive, off by default):

• A kv_transfer_only flag on a generation_only disaggregated request: the engine pulls the KV over
  the transceiver, commits it for reuse, and finishes without a generation forward pass.
• A fire-and-forget POST /v1/kv_transfer {model, messages|prompt, source} endpoint (+
  GET /v1/kv_transfer/stats) that orchestrates the existing context_only → generation_only ticket
  flow internally, so callers never handle disagg tickets.

It reuses the existing disagg transceiver — no new transport, and regular disagg/serving paths are
unchanged. A PR follows in #15736.

Alternatives

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Additional context

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Review PR #15736 first, then trace the existing context_only → generation_only ticket flow and disagg transceiver path. The work is done when the additive kv_transfer_only behavior and the POST /v1/kv_transfer plus GET /v1/kv_transfer/stats surfaces support proactive KV staging without a generation pass, while regular disaggregated serving remains unchanged.