Implement PrisKVConnector for vLLM-Omni KV Communication

@bob-021206 is already working on this.

Since Mar 17, 2026.

### 🚀 Feature Description and Motivation

This issue proposes adding a PrisKV-based connector for vLLM-Omni so that vLLM-Omni can use the PrisKV client for KV communication across pipeline stages.

Currently, vLLM-Omni supports pluggable connectors for inter-stage KV transfer, but it does not yet provide a native connector for PrisKV. Since AIBrix uses PrisKV as a core distributed KV communication and storage component, this integration is required to enable efficient KV transfer within the AIBrix multi-modal serving stack.

This work includes:
- building and validating the PrisKV dependency in the vLLM-Omni environment,
- implementing a `PrisKVConnector` in vLLM-Omni,
- wiring the connector to use the PrisKV client APIs for KV put/get communication,
- ensuring the connector can serve as a functional backend for KV transfer in omni-modal pipelines.

This feature will allow vLLM-Omni to communicate KV states through PrisKV directly, reducing integration gaps between vLLM-Omni and AIBrix.

Part of #1966

### Use Case

We need vLLM-Omni to communicate KV data through PrisKV so that intermediate KV states can be written and retrieved through a native `PrisKVConnector` backed by the PrisKV client.

This would make it possible to run vLLM-Omni with AIBrix infrastructure and provide the foundation for later optimizations.

### Proposed Solution

_No response_