Add a per-connection cache capability override (supportsPromptCaching,
cacheControlPassthrough) stored in provider_specific_data.cache, consulted
first by providerSupportsCaching() / providerHonorsOpenAIFormatCacheControl()
before falling back to the hardcoded CACHING_PROVIDERS name sets. Unblocks
prompt_cache_key injection, the compression cache-aware guard, and
cache_control passthrough for openai-compatible-chat-<uuid>-style custom
connections that can never match the hardcoded provider-name sets. Default
(no override) is byte-identical to current behavior.
6c8392fa45