Add 'last two messages' cache breakpoint strategy for Messages API (#312472)

* Add 'last two messages' cache breakpoint strategy for Messages API

Adds an experiment-gated alternative to the heuristic-based cache
breakpoint placement. Places cache_control on the last two merged
Anthropic messages instead of using prompt-tsx markers and
addCacheBreakpoints. Gated behind AnthropicCacheBreakpointsLastTwoMessages
(default false, onExp).

* Address PR review: handle pre-marked tail messages in addLastTwoMessagesCacheControl

* Remove 'advanced' tag from cacheBreakpoints.lastTwoMessages setting