Review follow-up on the #56798 salvage: the gate shipped fully dormant
(no provider profile sets supports_prompt_cache_key, no production
caller passes it, and no plain 'openai' profile exists to set it on) —
AGENTS.md rejects dead code wired in without E2E proof.
Activate the one endpoint where the field is first-class: exact-host
api.openai.com (OpenAI documents prompt_cache_key; GPT-5.6+ docs
recommend it for cache routing). Deliberately NOT substring matching —
Azure/OpenAI-compat endpoints may reject unknown fields and stay
opt-in via the flag. 4 new tests (imply + 3 spoof/proxy/Azure
negatives); mutation-checked (substring-weakened host check fails the
spoof tests).
78e2987e20