fix: preserve Responses cache salt

[franciscojavierarceo]

Summary

• Preserve the optional Responses cache_salt when parsing RequestPayload and serializing the normalized request sent upstream.
• Add a regression test proving that a client/internal salt survives RequestPayload::to_upstream_request.
• Initialize the new optional field in existing test, benchmark, and internal request literals.

Why

The gateway previously accepted JSON containing cache_salt but silently discarded the unknown field during deserialization. In a KV-aware llm-d deployment, llm-d folds the salt into its first canonical request key and vLLM includes it in APC block identity. Dropping the field weakens tenant isolation and prevents llm-d's salted request keys from matching vLLM's salted KV events.

This implementation is intentionally separate from the ADR and benchmark evidence in #65.

Test Plan

• cargo test -p agentic-server-core request_payload_forwards_cache_salt_upstream -- --nocapture
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• uvx pre-commit run --all-files