feat(P1/P2): 完成TDD开发及P1/P2设计文档

## 设计文档
- multi_role_permission_design: 多角色权限设计 (CONDITIONAL GO)
- audit_log_enhancement_design: 审计日志增强 (CONDITIONAL GO)
- routing_strategy_template_design: 路由策略模板 (CONDITIONAL GO)
- sso_saml_technical_research: SSO/SAML调研 (CONDITIONAL GO)
- compliance_capability_package_design: 合规能力包设计 (CONDITIONAL GO)

## TDD开发成果
- IAM模块: supply-api/internal/iam/ (111个测试)
- 审计日志模块: supply-api/internal/audit/ (40+测试)
- 路由策略模块: gateway/internal/router/ (33+测试)
- 合规能力包: gateway/internal/compliance/ + scripts/ci/compliance/

## 规范文档
- parallel_agent_output_quality_standards: 并行Agent产出质量规范
- project_experience_summary: 项目经验总结 (v2)
- 2026-04-02-p1-p2-tdd-execution-plan: TDD执行计划

## 评审报告
- 5个CONDITIONAL GO设计文档评审报告
- fix_verification_report: 修复验证报告
- full_verification_report: 全面质量验证报告
- tdd_module_quality_verification: TDD模块质量验证
- tdd_execution_summary: TDD执行总结

依据: Superpowers执行框架 + TDD规范

gateway/internal/router/fallback/fallback_test.go

@@ -0,0 +1,192 @@
+package fallback
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"lijiaoqiao/gateway/internal/router/strategy"
+)
+
+// TestFallback_Tier1_Success 测试Tier1可用时直接返回
+func TestFallback_Tier1_Success(t *testing.T) {
+	fb := NewFallbackHandler()
+
+	// 设置Tier1 provider
+	fb.tiers = []TierConfig{
+		{
+			Tier: 1,
+			Providers: []string{"ProviderA"},
+		},
+	}
+
+	// 创建mock router
+	fb.router = &MockFallbackRouter{
+		providers: map[string]*MockFallbackProvider{
+			"ProviderA": {
+				name:      "ProviderA",
+				available: true,
+			},
+		},
+	}
+
+	// 设置metrics
+	fb.metrics = &MockFallbackMetrics{}
+
+	req := &strategy.RoutingRequest{
+		Model:  "gpt-4",
+		UserID: "user123",
+	}
+
+	decision, err := fb.Handle(context.Background(), req)
+
+	assert.NoError(t, err)
+	assert.NotNil(t, decision)
+	assert.Equal(t, "ProviderA", decision.Provider, "Should select Tier1 provider")
+	assert.True(t, decision.TakeoverMark, "TakeoverMark should be true")
+}
+
+// TestFallback_Tier1_Fail_Tier2 测试Tier1失败时降级到Tier2
+func TestFallback_Tier1_Fail_Tier2(t *testing.T) {
+	fb := NewFallbackHandler()
+
+	// 设置多级tier
+	fb.tiers = []TierConfig{
+		{Tier: 1, Providers: []string{"ProviderA"}},
+		{Tier: 2, Providers: []string{"ProviderB"}},
+	}
+
+	// Tier1不可用，Tier2可用
+	fb.router = &MockFallbackRouter{
+		providers: map[string]*MockFallbackProvider{
+			"ProviderA": {
+				name:      "ProviderA",
+				available: false, // Tier1 不可用
+			},
+			"ProviderB": {
+				name:      "ProviderB",
+				available: true, // Tier2 可用
+			},
+		},
+	}
+
+	fb.metrics = &MockFallbackMetrics{}
+
+	req := &strategy.RoutingRequest{
+		Model:  "gpt-4",
+		UserID: "user123",
+	}
+
+	decision, err := fb.Handle(context.Background(), req)
+
+	assert.NoError(t, err)
+	assert.NotNil(t, decision)
+	assert.Equal(t, "ProviderB", decision.Provider, "Should fallback to Tier2")
+}
+
+// TestFallback_AllFail 测试全部失败返回错误
+func TestFallback_AllFail(t *testing.T) {
+	fb := NewFallbackHandler()
+
+	fb.tiers = []TierConfig{
+		{Tier: 1, Providers: []string{"ProviderA"}},
+		{Tier: 2, Providers: []string{"ProviderB"}},
+	}
+
+	// 所有provider都不可用
+	fb.router = &MockFallbackRouter{
+		providers: map[string]*MockFallbackProvider{
+			"ProviderA": {name: "ProviderA", available: false},
+			"ProviderB": {name: "ProviderB", available: false},
+		},
+	}
+
+	fb.metrics = &MockFallbackMetrics{}
+
+	req := &strategy.RoutingRequest{
+		Model:  "gpt-4",
+		UserID: "user123",
+	}
+
+	decision, err := fb.Handle(context.Background(), req)
+
+	assert.Error(t, err, "Should return error when all tiers fail")
+	assert.Nil(t, decision)
+}
+
+// TestFallback_RatelimitIntegration 测试Fallback与ratelimit集成
+func TestFallback_RatelimitIntegration(t *testing.T) {
+	fb := NewFallbackHandler()
+
+	fb.tiers = []TierConfig{
+		{Tier: 1, Providers: []string{"ProviderA"}},
+	}
+
+	fb.router = &MockFallbackRouter{
+		providers: map[string]*MockFallbackProvider{
+			"ProviderA": {
+				name:           "ProviderA",
+				available:      true,
+				rateLimitError: errors.New("rate limit exceeded"), // 触发ratelimit
+			},
+		},
+	}
+
+	fb.metrics = &MockFallbackMetrics{}
+
+	req := &strategy.RoutingRequest{
+		Model:  "gpt-4",
+		UserID: "user123",
+	}
+
+	_, err := fb.Handle(context.Background(), req)
+
+	// 应该检测到ratelimit错误并返回
+	assert.Error(t, err, "Should return error on rate limit")
+	assert.Contains(t, err.Error(), "rate limit", "Error should mention rate limit")
+}
+
+// MockFallbackRouter 用于测试的Mock Router
+type MockFallbackRouter struct {
+	providers map[string]*MockFallbackProvider
+}
+
+func (r *MockFallbackRouter) SelectProvider(ctx context.Context, req *strategy.RoutingRequest, providerName string) (*strategy.RoutingDecision, error) {
+	provider, ok := r.providers[providerName]
+	if !ok {
+		return nil, errors.New("provider not found")
+	}
+
+	if !provider.available {
+		return nil, errors.New("provider not available")
+	}
+
+	if provider.rateLimitError != nil {
+		return nil, provider.rateLimitError
+	}
+
+	return &strategy.RoutingDecision{
+		Provider:     providerName,
+		TakeoverMark: true,
+	}, nil
+}
+
+// MockFallbackProvider 用于测试的Mock Provider
+type MockFallbackProvider struct {
+	name           string
+	available      bool
+	rateLimitError error
+}
+
+// MockFallbackMetrics 用于测试的Mock Metrics
+type MockFallbackMetrics struct {
+	recordCalled bool
+	tier         int
+}
+
+func (m *MockFallbackMetrics) RecordTakeoverMark(provider string, tier int) {
+	m.recordCalled = true
+	m.tier = tier
+}