test(P0-5): 补齐 health handler 和 ticket stats handler 测试

新增 internal/http/handlers/health_handler_test.go:
- TestHealthHandler_Live_ReturnsUPWhenLive
- TestHealthHandler_Live_ReturnsDOWNWhenNotLive
- TestHealthHandler_Live_WithNilProbe
- TestHealthHandler_Ready_WithFailingChecker
- TestHealthHandler_Ready_WithPassingChecker
- TestHealthHandler_Health_ReturnsOK
- TestTicketStatsHandler_Get_Success
- TestTicketStatsHandler_Get_Error
- TestTicketStatsHandler_Get_NilAudit

**覆盖率提升**:
- internal/http/handlers: 78.4% → **84.4%** (+6.0%)
- 整体覆盖率: 74.8% → **76.3%** (+1.5%)

所有 P0 任务完成！Phase 2 测试补齐全部达成 🎉

Ref: test/PHASE2_TEST_PLAN.md P0-5

internal/http/handlers/health_handler_test.go

@@ -0,0 +1,180 @@
+package handlers
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/bridge/ai-customer-service/internal/domain/audit"
+	"github.com/bridge/ai-customer-service/internal/domain/ticketstats"
+	"github.com/bridge/ai-customer-service/internal/platform/health"
+)
+
+func TestHealthHandler_Live_ReturnsUPWhenLive(t *testing.T) {
+	probe := health.NewProbe()
+	probe.SetLive(true)
+	h := NewHealthHandler(probe)
+
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health/live", nil)
+	rr := httptest.NewRecorder()
+	h.Live(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Live() status = %d, want 200", rr.Code)
+	}
+}
+
+func TestHealthHandler_Live_ReturnsDOWNWhenNotLive(t *testing.T) {
+	probe := health.NewProbe()
+	probe.SetLive(false)
+	h := NewHealthHandler(probe)
+
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health/live", nil)
+	rr := httptest.NewRecorder()
+	h.Live(rr, req)
+	if rr.Code != http.StatusServiceUnavailable {
+		t.Errorf("Live() status = %d, want 503", rr.Code)
+	}
+}
+
+func TestHealthHandler_Live_WithNilProbe(t *testing.T) {
+	h := NewHealthHandler(nil)
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health/live", nil)
+	rr := httptest.NewRecorder()
+	h.Live(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Live() with nil probe status = %d, want 200", rr.Code)
+	}
+}
+
+func TestHealthHandler_Ready_WithFailingChecker(t *testing.T) {
+	probe := health.NewProbe()
+	probe.SetLive(true)
+	h := NewHealthHandler(probe, &failingHealthChecker{})
+
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health/ready", nil)
+	rr := httptest.NewRecorder()
+	h.Ready(rr, req)
+	if rr.Code != http.StatusServiceUnavailable {
+		t.Errorf("Ready() with failing checker status = %d, want 503", rr.Code)
+	}
+}
+
+func TestHealthHandler_Ready_WithPassingChecker(t *testing.T) {
+	probe := health.NewProbe()
+	probe.SetLive(true)
+	h := NewHealthHandler(probe, &passingHealthChecker{})
+
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health/ready", nil)
+	rr := httptest.NewRecorder()
+	h.Ready(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Ready() with passing checker status = %d, want 200", rr.Code)
+	}
+}
+
+func TestHealthHandler_Health_ReturnsOK(t *testing.T) {
+	probe := health.NewProbe()
+	probe.SetLive(true)
+	h := NewHealthHandler(probe)
+
+	req := httptest.NewRequest(http.MethodGet, "/actuator/health", nil)
+	rr := httptest.NewRecorder()
+	h.Health(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Health() status = %d, want 200", rr.Code)
+	}
+}
+
+// --- TicketStatsHandler tests ---
+
+func TestTicketStatsHandler_Get_Success(t *testing.T) {
+	mock := &mockTicketStatsServiceForStats{
+		stats: ticketstats.Stats{
+			Total:        100,
+			Open:         30,
+			Resolved:     50,
+			Closed:       20,
+			ByChannel:    map[string]int{"api": 40, "web": 60},
+			ByPriority:   map[string]int{"P1": 10, "P2": 60, "P3": 30},
+			HandoffCount: 15,
+			AvgResolutionTimeMinutes: 45.5,
+		},
+		err: nil,
+	}
+	recorder := &stubAuditRecorderForStats{}
+	h := NewTicketStatsHandler(mock, recorder)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/customer-service/tickets/stats", nil)
+	rr := httptest.NewRecorder()
+	h.Get(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Get() status = %d, want 200", rr.Code)
+	}
+}
+
+func TestTicketStatsHandler_Get_Error(t *testing.T) {
+	mock := &mockTicketStatsServiceForStats{
+		stats: ticketstats.Stats{},
+		err:   errStub{"stats error"},
+	}
+	recorder := &stubAuditRecorderForStats{}
+	h := NewTicketStatsHandler(mock, recorder)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/customer-service/tickets/stats", nil)
+	rr := httptest.NewRecorder()
+	h.Get(rr, req)
+	if rr.Code != http.StatusInternalServerError {
+		t.Errorf("Get() with error status = %d, want 500", rr.Code)
+	}
+}
+
+func TestTicketStatsHandler_Get_NilAudit(t *testing.T) {
+	mock := &mockTicketStatsServiceForStats{
+		stats: ticketstats.Stats{},
+		err:   nil,
+	}
+	h := NewTicketStatsHandler(mock, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/customer-service/tickets/stats", nil)
+	rr := httptest.NewRecorder()
+	h.Get(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Errorf("Get() with nil audit status = %d, want 200", rr.Code)
+	}
+}
+
+// --- Test doubles ---
+
+type passingHealthChecker struct{}
+
+func (c *passingHealthChecker) Name() string { return "passing" }
+
+func (c *passingHealthChecker) Check(ctx context.Context) error { return nil }
+
+type failingHealthChecker struct{}
+
+func (c *failingHealthChecker) Name() string { return "failing" }
+
+func (c *failingHealthChecker) Check(ctx context.Context) error {
+	return errStub{"checker failed"}
+}
+
+type errStub struct{ msg string }
+
+func (e errStub) Error() string { return e.msg }
+
+type mockTicketStatsServiceForStats struct {
+	stats ticketstats.Stats
+	err   error
+}
+
+func (m *mockTicketStatsServiceForStats) GetStats(ctx context.Context) (ticketstats.Stats, error) {
+	return m.stats, m.err
+}
+
+type stubAuditRecorderForStats struct{}
+
+func (s *stubAuditRecorderForStats) Add(ctx context.Context, event audit.Event) error {
+	return nil
+}