feat(v2): populate CacheHints on provider.Request when caching enabled

buildProviderRequest now computes cache-breakpoint positions automatically
when the WithPromptCaching() option is set. It places up to 3 hints:
tools, system, and the index of the last non-system message. Providers
that don't support caching (OpenAI, Google) ignore the field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/llm.go

@@ -122,6 +122,26 @@ func buildProviderRequest(model string, messages []Message, cfg *requestConfig)
 		}
 	}
 
+	if cfg.cacheConfig != nil && cfg.cacheConfig.enabled {
+		hints := &provider.CacheHints{LastCacheableMessageIndex: -1}
+		if len(req.Tools) > 0 {
+			hints.CacheTools = true
+		}
+		for _, m := range messages {
+			if m.Role == RoleSystem {
+				hints.CacheSystem = true
+				break
+			}
+		}
+		for i := len(messages) - 1; i >= 0; i-- {
+			if messages[i].Role != RoleSystem {
+				hints.LastCacheableMessageIndex = i
+				break
+			}
+		}
+		req.CacheHints = hints
+	}
+
 	return req
 }