Prompt caching on Gemini + Anthropic

[shreydevkar]

Long chain-mode turns re-send the full sheet context every time. Enable prompt caching on providers that support it (Anthropic beta, Gemini CachedContent) to cut latency and cost on the 2nd-plus turn.

Acceptance criteria:

• Provider abstraction gets a cache_id / cache_name field.
• System-prompt + context turn gets cached on the first call.
• Follow-up calls reference the cached prefix.