fix: only add cache_control to last tool in Anthropic prompt caching

[tomtrin]

Summary

• maybe_cache_tools/2 was adding cache_control: %{type: "ephemeral"} to every tool definition via Enum.map, exceeding Anthropic's 4 cache_control breakpoint limit with 3+ tools
• Since Anthropic's caching is prefix-based, only the last tool needs a breakpoint — it caches all preceding tools in the prefix
• Changed to use Enum.split(tools, -1) to only annotate the last tool

Before (5 tools + system + message = 7 breakpoints)

tool_a: cache_control ← redundant
tool_b: cache_control ← redundant
tool_c: cache_control ← redundant
tool_d: cache_control ← redundant
tool_e: cache_control
system: cache_control
message: cache_control
Total: 7 (exceeds limit of 4)

After (last tool + system + message = 3 breakpoints)

tool_a:
tool_b:
tool_c:
tool_d:
tool_e: cache_control
system: cache_control
message: cache_control
Total: 3 (within limit of 4)

Test plan

• Added test with 5 tools verifying only the last gets cache_control
• All 31 existing prompt cache tests pass unchanged

Fixes #452