test(P0-1): 补齐 memory store ticket workflow 测试

新增 internal/store/memory/ticket_workflow_test.go：
- TestTicketStore_ListOpen: 测试开放工单过滤逻辑
- TestTicketStore_Assign: 测试工单分配（正常+已分配冲突）
- TestTicketStore_Resolve: 测试工单解决
- TestTicketStore_Close: 测试工单关闭（正常+非resolved错误）

**覆盖率提升**:
- internal/store/memory: 59.1% → **88.3%** (+29.2%)
- 整体覆盖率: 62.6% → **66.2%** (+3.6%)

Ref: test/PHASE2_TEST_PLAN.md P0-1

internal/store/memory/ticket_workflow_test.go

@@ -0,0 +1,167 @@
+package memory
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/bridge/ai-customer-service/internal/domain/ticket"
+)
+
+func TestTicketStore_ListOpen(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create tickets with different statuses
+	tickets := []ticket.Ticket{
+		{ID: "t1", Status: ticket.StatusOpen, CreatedAt: now, UpdatedAt: now},
+		{ID: "t2", Status: ticket.StatusAssigned, CreatedAt: now, UpdatedAt: now},
+		{ID: "t3", Status: ticket.StatusResolved, CreatedAt: now, UpdatedAt: now},
+		{ID: "t4", Status: ticket.StatusClosed, CreatedAt: now, UpdatedAt: now},
+	}
+	for i := range tickets {
+		store.Create(ctx, &tickets[i])
+	}
+
+	// ListOpen should return open + assigned (not resolved/closed)
+	open, err := store.ListOpen(ctx, 10)
+	if err != nil {
+		t.Fatalf("ListOpen() error = %v", err)
+	}
+	if len(open) != 2 {
+		t.Errorf("ListOpen() len = %d, want 2 (open + assigned)", len(open))
+	}
+}
+
+func TestTicketStore_Assign(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create an open ticket
+	store.Create(ctx, &ticket.Ticket{
+		ID:     "t1",
+		Status: ticket.StatusOpen,
+		CreatedAt: now,
+		UpdatedAt: now,
+	})
+
+	// Assign it
+	err := store.Assign(ctx, "t1", "agent1", "admin", "127.0.0.1", now)
+	if err != nil {
+		t.Fatalf("Assign() error = %v", err)
+	}
+
+	// Verify assigned
+	tkt, _ := store.GetByID(ctx, "t1")
+	if tkt.Status != ticket.StatusAssigned {
+		t.Errorf("ticket.Status = %s, want %s", tkt.Status, ticket.StatusAssigned)
+	}
+	if tkt.AssignedTo != "agent1" {
+		t.Errorf("ticket.AssignedTo = %s, want agent1", tkt.AssignedTo)
+	}
+}
+
+func TestTicketStore_Assign_AlreadyAssigned(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create an already-assigned ticket
+	store.Create(ctx, &ticket.Ticket{
+		ID:         "t1",
+		Status:     ticket.StatusAssigned,
+		AssignedTo: "agent1",
+		CreatedAt:  now,
+		UpdatedAt:  now,
+	})
+
+	// Try to assign again — should return error
+	err := store.Assign(ctx, "t1", "agent2", "admin", "127.0.0.1", now)
+	if err == nil {
+		t.Fatal("Assign() on already-assigned ticket should return error")
+	}
+}
+
+func TestTicketStore_Resolve(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create an assigned ticket
+	store.Create(ctx, &ticket.Ticket{
+		ID:     "t1",
+		Status: ticket.StatusAssigned,
+		AssignedTo: "agent1",
+		CreatedAt: now,
+		UpdatedAt: now,
+	})
+
+	// Resolve it
+	err := store.Resolve(ctx, "t1", "fixed by agent", "admin", "127.0.0.1", now)
+	if err != nil {
+		t.Fatalf("Resolve() error = %v", err)
+	}
+
+	// Verify resolved
+	tkt, _ := store.GetByID(ctx, "t1")
+	if tkt.Status != ticket.StatusResolved {
+		t.Errorf("ticket.Status = %s, want %s", tkt.Status, ticket.StatusResolved)
+	}
+	if tkt.Resolution != "fixed by agent" {
+		t.Errorf("ticket.Resolution = %s, want 'fixed by agent'", tkt.Resolution)
+	}
+	if tkt.ResolvedAt == nil {
+		t.Error("ticket.ResolvedAt should be set")
+	}
+}
+
+func TestTicketStore_Close(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create a resolved ticket
+	resolvedTime := now.Add(-1 * time.Hour)
+	store.Create(ctx, &ticket.Ticket{
+		ID:         "t1",
+		Status:     ticket.StatusResolved,
+		Resolution: "fixed",
+		ResolvedAt: &resolvedTime,
+		CreatedAt:  now,
+		UpdatedAt:  now,
+	})
+
+	// Close it
+	err := store.Close(ctx, "t1", "user confirmed", "admin", "127.0.0.1", now)
+	if err != nil {
+		t.Fatalf("Close() error = %v", err)
+	}
+
+	// Verify closed
+	tkt, _ := store.GetByID(ctx, "t1")
+	if tkt.Status != ticket.StatusClosed {
+		t.Errorf("ticket.Status = %s, want %s", tkt.Status, ticket.StatusClosed)
+	}
+}
+
+func TestTicketStore_Close_NotResolved(t *testing.T) {
+	store := NewTicketStore()
+	ctx := context.Background()
+	now := time.Now().Truncate(time.Second)
+
+	// Create an open ticket (not resolved)
+	store.Create(ctx, &ticket.Ticket{
+		ID:     "t1",
+		Status: ticket.StatusOpen,
+		CreatedAt: now,
+		UpdatedAt: now,
+	})
+
+	// Try to close — should return error
+	err := store.Close(ctx, "t1", "user confirmed", "admin", "127.0.0.1", now)
+	if err == nil {
+		t.Fatal("Close() on non-resolved ticket should return error")
+	}
+}