is_openai_prompt_cache_key_enabled() -> boolReturn whether OpenAI model calls should carry a per-thread cache key.
Resolves the models.openai_prompt_cache_key option from env/config.toml,
defaulting to enabled. When disabled, ConfigurableModelMiddleware stops
injecting the thread ID as an OpenAI prompt_cache_key (a user-supplied key
is still forwarded). This is the opt-out for OpenAI-compatible endpoints that
reject unknown request fields.