refactor(v2/anthropic): use MultiSystem for system prompts

Switches buildRequest to emit anthReq.MultiSystem instead of anthReq.System
whenever a system message is present. Upstream's MarshalJSON prefers
MultiSystem when non-empty, so the wire format is unchanged for requests
without cache_control. This refactor is a prerequisite for attaching
cache_control markers to system parts in the next commit.

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

v2/anthropic/anthropic.go

@@ -82,13 +82,14 @@ func (p *Provider) buildRequest(req provider.Request) anth.MessagesRequest {
 	}
 
 	var msgs []anth.Message
+	var systemText string
 
 	for _, msg := range req.Messages {
 		if msg.Role == "system" {
-			if len(anthReq.System) > 0 {
-				anthReq.System += "\n"
+			if len(systemText) > 0 {
+				systemText += "\n"
 			}
-			anthReq.System += msg.Content
+			systemText += msg.Content
 			continue
 		}
 
@@ -224,6 +225,12 @@ func (p *Provider) buildRequest(req provider.Request) anth.MessagesRequest {
 
 	anthReq.Messages = msgs
 
+	if systemText != "" {
+		anthReq.MultiSystem = []anth.MessageSystemPart{
+			anth.NewSystemMessagePart(systemText),
+		}
+	}
+
 	if req.Temperature != nil {
 		f := float32(*req.Temperature)
 		anthReq.Temperature = &f