Avoid intermediate conversation history copies

internal/domain/exchange_test.go

@@ -51,3 +51,25 @@ func TestExchangeManyTurns(t *testing.T) {
 		}
 	}
 }
+
+func TestExchangeMessagesWithSuffix(t *testing.T) {
+	for _, history := range []*Exchange{nil, CaptureExchange(nil, "source", nil, "answer")} {
+		control := &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}
+		first := []RenderedMessage{{Role: RoleUser, Content: "new", CacheControl: control}}
+		last := []RenderedMessage{{Role: RoleDeveloper, Content: "suffix"}}
+		got := history.MessagesWithSuffix(first, nil, last)
+		want := append(history.Messages(), first...)
+		want = append(want, last...)
+		if !reflect.DeepEqual(got, want) {
+			t.Fatalf("messages = %#v, want %#v", got, want)
+		}
+		got[history.MessageCount()].CacheControl.TTL = "changed"
+		got[history.MessageCount()].Content = "changed"
+		if first[0].Content != "new" || control.TTL != "1h" {
+			t.Fatal("assembled messages alias the caller suffix")
+		}
+		if history != nil && history.Messages()[0].Content != "answer" {
+			t.Fatal("assembly changed retained history")
+		}
+	}
+}