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

v2/anthropic/cache_test.go

@@ -0,0 +1,84 @@
+package anthropic
+
+import (
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/go-llm/v2/provider"
+	anth "github.com/liushuangls/go-anthropic/v2"
+)
+
+// TestBuildRequest_MultiSystemUsedWhenSystemPresent verifies that after the
+// refactor, the Anthropic provider uses MultiSystem (multi-part) rather than
+// the flat System string when a system message is present. This is
+// behavior-preserving — the upstream client's MarshalJSON prefers
+// MultiSystem when both are set.
+func TestBuildRequest_MultiSystemUsedWhenSystemPresent(t *testing.T) {
+	p := New("test-key")
+	req := provider.Request{
+		Model: "claude-sonnet-4-6",
+		Messages: []provider.Message{
+			{Role: "system", Content: "you are helpful"},
+			{Role: "user", Content: "hello"},
+		},
+	}
+	anthReq := p.buildRequest(req)
+
+	if len(anthReq.MultiSystem) != 1 {
+		t.Fatalf("expected 1 MultiSystem part, got %d", len(anthReq.MultiSystem))
+	}
+	if anthReq.MultiSystem[0].Text != "you are helpful" {
+		t.Errorf("expected MultiSystem text 'you are helpful', got %q", anthReq.MultiSystem[0].Text)
+	}
+	if anthReq.MultiSystem[0].Type != "text" {
+		t.Errorf("expected MultiSystem type 'text', got %q", anthReq.MultiSystem[0].Type)
+	}
+	if anthReq.System != "" {
+		t.Errorf("expected System string to be empty when MultiSystem is used, got %q", anthReq.System)
+	}
+}
+
+// TestBuildRequest_MultipleSystemMessagesConcatenated verifies that multiple
+// system messages are joined into a single MultiSystem part (preserving
+// existing newline-joined behavior from the old code).
+func TestBuildRequest_MultipleSystemMessagesConcatenated(t *testing.T) {
+	p := New("test-key")
+	req := provider.Request{
+		Model: "claude-sonnet-4-6",
+		Messages: []provider.Message{
+			{Role: "system", Content: "part A"},
+			{Role: "system", Content: "part B"},
+			{Role: "user", Content: "hello"},
+		},
+	}
+	anthReq := p.buildRequest(req)
+
+	if len(anthReq.MultiSystem) != 1 {
+		t.Fatalf("expected 1 MultiSystem part after concat, got %d", len(anthReq.MultiSystem))
+	}
+	expected := "part A\npart B"
+	if anthReq.MultiSystem[0].Text != expected {
+		t.Errorf("expected MultiSystem text %q, got %q", expected, anthReq.MultiSystem[0].Text)
+	}
+}
+
+// TestBuildRequest_NoSystemMessage verifies that when there's no system
+// message, both System and MultiSystem are empty.
+func TestBuildRequest_NoSystemMessage(t *testing.T) {
+	p := New("test-key")
+	req := provider.Request{
+		Model: "claude-sonnet-4-6",
+		Messages: []provider.Message{
+			{Role: "user", Content: "hello"},
+		},
+	}
+	anthReq := p.buildRequest(req)
+
+	if len(anthReq.MultiSystem) != 0 {
+		t.Errorf("expected empty MultiSystem when no system message, got %d parts", len(anthReq.MultiSystem))
+	}
+	if anthReq.System != "" {
+		t.Errorf("expected empty System string, got %q", anthReq.System)
+	}
+	// Silences the unused import warning in this file until Task 5.
+	_ = anth.CacheControlTypeEphemeral
+}