Improve caching

[dgageot]

• Cut the message that contained the date, env, agents.md and skills into separate system messages
• Reordered the system messages to have the most invariant messages first
• Instead of caching the first two system messages and the last two user/assistant messages, we mark the tool instructions with a cache control checkpoint because it's the last system message before the ones that can be more user specific
• We mark the last system message too.
• We keep on marking the last user/assistant messages with the cache control flag.

Added a few tests and e2e on the way.

[dgageot]

/review