feat: refactor HandoffGraph to use structured arguments and add unit tests for buildReason method

internal/ai/runtime/graphs/handoff_graph_test.go

@@ -0,0 +1,31 @@
+package graphs
+
+import (
+	"testing"
+
+	"cs-agent/internal/models"
+)
+
+func TestHandoffGraphBuildReason(t *testing.T) {
+	graph := NewHandoffGraph(&models.Conversation{ID: 1}, &models.AIAgent{Name: "AI"})
+
+	reason, err := graph.buildReason(`{"reason":"  用户需要人工确认  "}`)
+	if err != nil {
+		t.Fatalf("buildReason returned error: %v", err)
+	}
+	if reason != "用户需要人工确认" {
+		t.Fatalf("unexpected reason: %q", reason)
+	}
+}
+
+func TestHandoffGraphBuildReasonFallback(t *testing.T) {
+	graph := NewHandoffGraph(&models.Conversation{ID: 1}, &models.AIAgent{Name: "AI"})
+
+	reason, err := graph.buildReason(`{}`)
+	if err != nil {
+		t.Fatalf("buildReason returned error: %v", err)
+	}
+	if reason != "用户需要转人工支持" {
+		t.Fatalf("unexpected fallback reason: %q", reason)
+	}
+}