Implements unified model routing

Migrates the AMP module to a new unified routing system, replacing the fallback handler with a router-based approach.

This change introduces a `ModelRoutingWrapper` that handles model extraction, routing decisions, and proxying based on provider availability and model mappings.
It provides a more flexible and maintainable routing mechanism by centralizing routing logic.

The changes include:
- Introducing new `routing` package with core routing logic.
- Creating characterization tests to capture existing behavior.
- Implementing model extraction and rewriting.
- Updating AMP module routes to utilize the new routing wrapper.
- Deprecating `FallbackHandler` in favor of the new `ModelRoutingWrapper`.

internal/routing/router_v2_test.go

@@ -0,0 +1,245 @@
+package routing
+
+import (
+	"context"
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRouter_DefaultMode_PrefersLocal(t *testing.T) {
+	// Setup: Create a router with a mock provider that supports "gpt-4"
+	registry := NewRegistry()
+	mockProvider := &MockProvider{
+		name:            "openai",
+		supportedModels: []string{"gpt-4"},
+		available:       true,
+		priority:        1,
+	}
+	registry.Register(mockProvider)
+
+	cfg := &config.Config{
+		AmpCode: config.AmpCode{
+			ModelMappings: []config.AmpModelMapping{
+				{From: "gpt-4", To: "claude-local"},
+			},
+		},
+	}
+
+	router := NewRouter(registry, cfg)
+
+	// Test: Request gpt-4 when local provider exists
+	req := RoutingRequest{
+		RequestedModel:      "gpt-4",
+		PreferLocalProvider: true,
+		ForceModelMapping:   false,
+	}
+
+	decision := router.ResolveV2(req)
+
+	// Assert: Should return LOCAL_PROVIDER, not MODEL_MAPPING
+	assert.Equal(t, RouteTypeLocalProvider, decision.RouteType)
+	assert.Equal(t, "gpt-4", decision.ResolvedModel)
+	assert.Equal(t, "openai", decision.ProviderName)
+	assert.False(t, decision.ShouldProxy)
+}
+
+func TestRouter_DefaultMode_MapsWhenNoLocal(t *testing.T) {
+	// Setup: Create a router with NO provider for "gpt-4" but a mapping to "claude-local"
+	// which has a provider
+	registry := NewRegistry()
+	mockProvider := &MockProvider{
+		name:            "anthropic",
+		supportedModels: []string{"claude-local"},
+		available:       true,
+		priority:        1,
+	}
+	registry.Register(mockProvider)
+
+	cfg := &config.Config{
+		AmpCode: config.AmpCode{
+			ModelMappings: []config.AmpModelMapping{
+				{From: "gpt-4", To: "claude-local"},
+			},
+		},
+	}
+
+	router := NewRouter(registry, cfg)
+
+	// Test: Request gpt-4 when no local provider exists, but mapping exists
+	req := RoutingRequest{
+		RequestedModel:      "gpt-4",
+		PreferLocalProvider: true,
+		ForceModelMapping:   false,
+	}
+
+	decision := router.ResolveV2(req)
+
+	// Assert: Should return MODEL_MAPPING
+	assert.Equal(t, RouteTypeModelMapping, decision.RouteType)
+	assert.Equal(t, "claude-local", decision.ResolvedModel)
+	assert.Equal(t, "anthropic", decision.ProviderName)
+	assert.False(t, decision.ShouldProxy)
+}
+
+func TestRouter_DefaultMode_AmpCreditsWhenNoLocalOrMapping(t *testing.T) {
+	// Setup: Create a router with no providers and no mappings
+	registry := NewRegistry()
+
+	cfg := &config.Config{
+		AmpCode: config.AmpCode{
+			ModelMappings: []config.AmpModelMapping{},
+		},
+	}
+
+	router := NewRouter(registry, cfg)
+
+	// Test: Request a model with no local provider and no mapping
+	req := RoutingRequest{
+		RequestedModel:      "unknown-model",
+		PreferLocalProvider: true,
+		ForceModelMapping:   false,
+	}
+
+	decision := router.ResolveV2(req)
+
+	// Assert: Should return AMP_CREDITS with ShouldProxy=true
+	assert.Equal(t, RouteTypeAmpCredits, decision.RouteType)
+	assert.Equal(t, "unknown-model", decision.ResolvedModel)
+	assert.True(t, decision.ShouldProxy)
+	assert.Empty(t, decision.ProviderName)
+}
+
+func TestRouter_ForceMode_MapsEvenWithLocal(t *testing.T) {
+	// Setup: Create a router with BOTH a local provider for "gpt-4" AND a mapping from "gpt-4" to "claude-local"
+	// The mapping target "claude-local" also has a provider
+	registry := NewRegistry()
+
+	// Local provider for gpt-4
+	openaiProvider := &MockProvider{
+		name:            "openai",
+		supportedModels: []string{"gpt-4"},
+		available:       true,
+		priority:        1,
+	}
+	registry.Register(openaiProvider)
+
+	// Local provider for the mapped model
+	anthropicProvider := &MockProvider{
+		name:            "anthropic",
+		supportedModels: []string{"claude-local"},
+		available:       true,
+		priority:        2,
+	}
+	registry.Register(anthropicProvider)
+
+	cfg := &config.Config{
+		AmpCode: config.AmpCode{
+			ModelMappings: []config.AmpModelMapping{
+				{From: "gpt-4", To: "claude-local"},
+			},
+		},
+	}
+
+	router := NewRouter(registry, cfg)
+
+	// Test: Request gpt-4 with ForceModelMapping=true
+	// Even though gpt-4 has a local provider, mapping should take precedence
+	req := RoutingRequest{
+		RequestedModel:      "gpt-4",
+		PreferLocalProvider: false,
+		ForceModelMapping:   true,
+	}
+
+	decision := router.ResolveV2(req)
+
+	// Assert: Should return MODEL_MAPPING, not LOCAL_PROVIDER
+	assert.Equal(t, RouteTypeModelMapping, decision.RouteType)
+	assert.Equal(t, "claude-local", decision.ResolvedModel)
+	assert.Equal(t, "anthropic", decision.ProviderName)
+	assert.False(t, decision.ShouldProxy)
+}
+
+func TestRouter_ThinkingSuffix_Preserved(t *testing.T) {
+	// Setup: Create a router with mapping and provider for mapped model
+	registry := NewRegistry()
+
+	mockProvider := &MockProvider{
+		name:            "anthropic",
+		supportedModels: []string{"claude-local"},
+		available:       true,
+		priority:        1,
+	}
+	registry.Register(mockProvider)
+
+	cfg := &config.Config{
+		AmpCode: config.AmpCode{
+			ModelMappings: []config.AmpModelMapping{
+				{From: "claude-3-5-sonnet", To: "claude-local"},
+			},
+		},
+	}
+
+	router := NewRouter(registry, cfg)
+
+	// Test: Request claude-3-5-sonnet with thinking suffix
+	req := RoutingRequest{
+		RequestedModel:      "claude-3-5-sonnet(thinking:foo)",
+		PreferLocalProvider: true,
+		ForceModelMapping:   false,
+	}
+
+	decision := router.ResolveV2(req)
+
+	// Assert: Thinking suffix should be preserved in resolved model
+	assert.Equal(t, RouteTypeModelMapping, decision.RouteType)
+	assert.Equal(t, "claude-local(thinking:foo)", decision.ResolvedModel)
+	assert.Equal(t, "anthropic", decision.ProviderName)
+}
+
+// MockProvider is a mock implementation of Provider for testing
+type MockProvider struct {
+	name            string
+	providerType    ProviderType
+	supportedModels []string
+	available       bool
+	priority        int
+}
+
+func (m *MockProvider) Name() string {
+	return m.name
+}
+
+func (m *MockProvider) Type() ProviderType {
+	if m.providerType == "" {
+		return ProviderTypeOAuth
+	}
+	return m.providerType
+}
+
+func (m *MockProvider) SupportsModel(model string) bool {
+	for _, supported := range m.supportedModels {
+		if supported == model {
+			return true
+		}
+	}
+	return false
+}
+
+func (m *MockProvider) Available(model string) bool {
+	return m.available
+}
+
+func (m *MockProvider) Priority() int {
+	return m.priority
+}
+
+func (m *MockProvider) Execute(ctx context.Context, model string, req executor.Request) (executor.Response, error) {
+	return executor.Response{}, nil
+}
+
+func (m *MockProvider) ExecuteStream(ctx context.Context, model string, req executor.Request) (<-chan executor.StreamChunk, error) {
+	return nil, nil
+}