feat(anthropic): Add support for automatic caching

[satoved]

Problem

Prism's Anthropic provider only supports explicit cache breakpoints — where you manually place cache_control on individual content blocks (system messages, user messages, tools). There's no way to use Anthropic's automatic caching, which accepts a single top-level cache_control field on the request body and automatically applies caching to the last cacheable block — the simplest approach for multi-turn conversations.

Solution

Pass cache_control as a provider option (withProviderOptions(['cache_control' => ['type' => 'ephemeral']])) and forward it as a top-level field in the Anthropic API payload for both Text and handlers. This lets users opt into automatic caching with a single line, without changing how explicit breakpoints work.

Breaking Changes

None.