Upstream exact Responses prefix routing through llm-d

Parent tracker: #69
Design and measured evidence: #65

Summary

Deliver the first and highest-impact milestone: exact Responses-aware KV-affine placement through llm-d after agentic-api has rehydrated previous_response_id state.

Scope

• Upstream vLLM support for accepting ResponsesRequest input at /tokenize and delegating to the same renderer used by inference.
• Upstream llm-d-router support for dispatching Responses token production to vLLM /tokenize, preserving cache_salt, and rejecting unresolved or incomplete cache identities.
• Propagate file-discovery rankIndex into endpoint metadata so multiple replicas on one host subscribe to distinct KV-event ports.
• Configure agentic-api's execution LLM URL to point at llm-d after rehydration rather than routing only on marginal client input.
• Keep llm-d canonical request keys distinct from vLLM engine block hashes while preserving their event-derived mapping.
• Verify model, tokenizer, template/parser, salt, LoRA, multimodal identity, block size, and key-format compatibility.
• Add metrics for matched blocks, selected endpoint, queue/load tradeoff, wrong-pod outcome, and vLLM cached input tokens.

Current evidence

The patched two-replica GPT-OSS-20B proof in #65 produced 12/12 warm returns and 99.08% mean continuation cache hits for both approximate and precise routing, versus 6/12 and 49.54% for load-only. Precise routing reduced mean continuation latency from 811.0 ms to 316.5 ms.

The proof used patched branch images. The vLLM and llm-d-router changes are not yet available in their upstream main branches.

Acceptance criteria

• Upstream-supported vLLM and llm-d-router images reproduce exact Responses token parity and KV-aware placement without benchmark-only patches.
• Precise routing materially improves cached tokens and continuation latency over load-only routing under concurrent agent/tool traffic and cache pressure.
• Different tenant salts cannot share KV blocks; identical permitted salts retain reuse.
• Wrong-pod selection, event lag, eviction, restart, and AllBlocksCleared are correctness-safe and observable.
• Same-host and multi-host replica discovery subscribe to the correct KV-event source for every endpoint.
• Agentic-api does not implement model tokenization or llm-d's canonical hash algorithm.