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/adapter.go

@@ -0,0 +1,39 @@
+// Package routing provides adapter to integrate with existing codebase.
+package routing
+
+import (
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
+)
+
+// Adapter bridges the new routing layer with existing auth manager.
+type Adapter struct {
+	router *Router
+	exec   *Executor
+}
+
+// NewAdapter creates a new adapter with the given configuration and auth manager.
+func NewAdapter(cfg *config.Config, authManager *coreauth.Manager) *Adapter {
+	registry := NewRegistry()
+	
+	// TODO: Register OAuth providers from authManager
+	// TODO: Register API key providers from cfg
+	
+	router := NewRouter(registry, cfg)
+	exec := NewExecutor(router)
+	
+	return &Adapter{
+		router: router,
+		exec:   exec,
+	}
+}
+
+// Router returns the underlying router.
+func (a *Adapter) Router() *Router {
+	return a.router
+}
+
+// Executor returns the underlying executor.
+func (a *Adapter) Executor() *Executor {
+	return a.exec
+}