Carry over #82973 and fix#81281 by preserving explicit cacheRetention for OpenAI-compatible completions providers that opt into prompt-cache-key support.
The change keeps explicit cacheRetention suppressed for OpenAI-compatible providers without compat.supportsPromptCacheKey, adds regression coverage for both paths, and updates prompt-caching docs for prompt_cache_key / prompt_cache_retention behavior.
Fixes#81281.
Supersedes #82973.
Co-authored-by: lonexreb <reach2shubhankar@gmail.com>