feat(routing): implement unified model routing with OAuth and API key providers

- Added a new routing package to manage provider registration and model resolution.
- Introduced Router, Executor, and Provider interfaces to handle different provider types.
- Implemented OAuthProvider and APIKeyProvider to support OAuth and API key authentication.
- Enhanced DefaultModelMapper to include OAuth model alias handling and fallback mechanisms.
- Updated context management in API handlers to preserve fallback models.
- Added tests for routing logic and provider selection.
- Enhanced Claude request conversion to handle reasoning content based on thinking mode.

internal/routing/provider.go

@@ -0,0 +1,80 @@
+// Package routing provides unified model routing for all provider types.
+package routing
+
+import (
+	"context"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+)
+
+// ProviderType indicates the type of provider.
+type ProviderType string
+
+const (
+	ProviderTypeOAuth  ProviderType = "oauth"
+	ProviderTypeAPIKey ProviderType = "api_key"
+	ProviderTypeVertex ProviderType = "vertex"
+)
+
+// Provider is the unified interface for all provider types (OAuth, API key, etc.).
+type Provider interface {
+	// Name returns the unique provider identifier.
+	Name() string
+
+	// Type returns the provider type.
+	Type() ProviderType
+
+	// SupportsModel returns true if this provider can handle the given model.
+	SupportsModel(model string) bool
+
+	// Available returns true if the provider is available for the model (not quota exceeded).
+	Available(model string) bool
+
+	// Priority returns the priority for this provider (lower = tried first).
+	Priority() int
+
+	// Execute sends the request to the provider.
+	Execute(ctx context.Context, model string, req executor.Request) (executor.Response, error)
+
+	// ExecuteStream sends a streaming request to the provider.
+	ExecuteStream(ctx context.Context, model string, req executor.Request) (<-chan executor.StreamChunk, error)
+}
+
+// ProviderCandidate represents a provider + model combination to try.
+type ProviderCandidate struct {
+	Provider Provider
+	Model    string // The actual model name to use (may be different from requested due to aliasing)
+}
+
+// Registry manages all available providers.
+type Registry struct {
+	providers []Provider
+}
+
+// NewRegistry creates a new provider registry.
+func NewRegistry() *Registry {
+	return &Registry{
+		providers: make([]Provider, 0),
+	}
+}
+
+// Register adds a provider to the registry.
+func (r *Registry) Register(p Provider) {
+	r.providers = append(r.providers, p)
+}
+
+// FindProviders returns all providers that support the given model and are available.
+func (r *Registry) FindProviders(model string) []Provider {
+	var result []Provider
+	for _, p := range r.providers {
+		if p.SupportsModel(model) && p.Available(model) {
+			result = append(result, p)
+		}
+	}
+	return result
+}
+
+// All returns all registered providers.
+func (r *Registry) All() []Provider {
+	return r.providers
+}