chore(llm): make cache: 'auto' the default (#26798)

packages/llm/test/provider/anthropic-messages-cache.recorded.test.ts

@@ -20,6 +20,9 @@ const cacheRequest = LLM.request({
   model,
   system: [{ type: "text", text: LARGE_CACHEABLE_SYSTEM, cache: new CacheHint({ type: "ephemeral" }) }],
   prompt: "Say hi.",
+  // Manual hint on the system part is the only marker we want here — skip the
+  // auto-policy's latest-user-message breakpoint so the cassette body matches.
+  cache: "none",
   generation: { maxTokens: 16, temperature: 0 },
 })