Gemini GenerateContent explicit cached-content adapter.

The adapter stores provider request prefixes, never model responses. It is deliberately usable only with the stable gemini-3.1-flash-lite model. The opaque binary returned from create/2 is a local private descriptor; callers must use cached_content_name/2 immediately before a Gemini request. Neither that descriptor nor Google's cachedContents/* name is suitable for events, logs, diagnostics, or persisted configuration.

Transport configuration is read from the erlang_adk application environment key gemini_context_cache. The value is a strict map with optional base_url, api_key, and request_timeout_ms keys. api_key falls back to GEMINI_API_KEY. Validation errors never echo credentials.

Google's current model page declares caching support for 3.1 Flash-Lite, but the current caching guide does not publish an explicit-cache minimum for this model. We therefore use a conservative 4096 estimated-token local floor and still treat the service response as authoritative.