[KVEvents] add dp rank to KV events

[sagearc]

atm llm-d-router reads dp rank from emitted kv event batches but drops it during adapter conversion.
kv events are instead attributed to the source ip:port attached to each subscriber. this is only sufficient for external load balancing where each dp rank exposes standalone frontend with a distinct port.

with internal load balancing, multiple dp ranks can share one http endpoint, so the endpoint alone cannot identify which rank owns the cached blocks.

refs:

• feat: add dp-rank to KV events sgl-project/sglang#6852
• feat: add data parallel rank to KVEventBatch vllm-project/vllm#18925

[Copilot]

Pull request overview

This PR preserves the data-parallel (DP) rank from incoming KV event batches by threading it through the engine adapters into kvevents.EventBatch, enabling downstream components to correctly attribute KV events when multiple ranks share a single HTTP endpoint.

Changes:

• Add optional DataParallelRank to kvevents.EventBatch.
• Propagate DP rank during msgpack batch decoding in both vLLM and SGLang engine adapters.
• Consolidate the shared msgpack batch struct into engineadapter/common.go and extend adapter tests to assert DP rank parsing.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/kvevents/events.go	Adds DataParallelRank to the public EventBatch type.
pkg/kvevents/engineadapter/common.go	Introduces shared msgpackEventBatch with optional DP rank field.
pkg/kvevents/engineadapter/vllm_adapter.go	Decodes DP rank from batch and sets it on kvevents.EventBatch.
pkg/kvevents/engineadapter/vllm_adapter_test.go	Updates test payload and asserts DP rank is preserved.
pkg/kvevents/engineadapter/sglang_adapter.go	Decodes DP rank from batch and sets it on kvevents.EventBatch.
pkg/kvevents/engineadapter/sglang_adapter_test.go	Updates test payload and asserts DP rank is preserved.

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