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/providers/oauth.go

@@ -0,0 +1,132 @@
+package providers
+
+import (
+	"context"
+	"errors"
+	"sync"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/routing"
+	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+)
+
+// OAuthProvider wraps OAuth-based auths as routing.Provider.
+type OAuthProvider struct {
+	name      string
+	auths     []*coreauth.Auth
+	mu        sync.RWMutex
+	executor  coreauth.ProviderExecutor
+}
+
+// NewOAuthProvider creates a new OAuth provider.
+func NewOAuthProvider(name string, exec coreauth.ProviderExecutor) *OAuthProvider {
+	return &OAuthProvider{
+		name:     name,
+		auths:    make([]*coreauth.Auth, 0),
+		executor: exec,
+	}
+}
+
+// Name returns the provider name.
+func (p *OAuthProvider) Name() string {
+	return p.name
+}
+
+// Type returns ProviderTypeOAuth.
+func (p *OAuthProvider) Type() routing.ProviderType {
+	return routing.ProviderTypeOAuth
+}
+
+// SupportsModel checks if any auth supports the model.
+func (p *OAuthProvider) SupportsModel(model string) bool {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	// OAuth providers typically support models via oauth-model-alias
+	// The actual model support is determined at execution time
+	return true
+}
+
+// Available checks if there's an available auth for the model.
+func (p *OAuthProvider) Available(model string) bool {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	for _, auth := range p.auths {
+		if p.isAuthAvailable(auth, model) {
+			return true
+		}
+	}
+	return false
+}
+
+// Priority returns the priority (OAuth is preferred over API key).
+func (p *OAuthProvider) Priority() int {
+	return 10
+}
+
+// Execute sends the request using an available OAuth auth.
+func (p *OAuthProvider) Execute(ctx context.Context, model string, req executor.Request) (executor.Response, error) {
+	auth := p.selectAuth(model)
+	if auth == nil {
+		return executor.Response{}, ErrNoAvailableAuth
+	}
+
+	return p.executor.Execute(ctx, auth, req, executor.Options{})
+}
+
+// ExecuteStream sends a streaming request.
+func (p *OAuthProvider) ExecuteStream(ctx context.Context, model string, req executor.Request) (<-chan executor.StreamChunk, error) {
+	auth := p.selectAuth(model)
+	if auth == nil {
+		return nil, ErrNoAvailableAuth
+	}
+
+	return p.executor.ExecuteStream(ctx, auth, req, executor.Options{})
+}
+
+// AddAuth adds an auth to this provider.
+func (p *OAuthProvider) AddAuth(auth *coreauth.Auth) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.auths = append(p.auths, auth)
+}
+
+// RemoveAuth removes an auth from this provider.
+func (p *OAuthProvider) RemoveAuth(authID string) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	filtered := make([]*coreauth.Auth, 0, len(p.auths))
+	for _, auth := range p.auths {
+		if auth.ID != authID {
+			filtered = append(filtered, auth)
+		}
+	}
+	p.auths = filtered
+}
+
+// isAuthAvailable checks if an auth is available for the model.
+func (p *OAuthProvider) isAuthAvailable(auth *coreauth.Auth, model string) bool {
+	// TODO: integrate with model_registry for quota checking
+	// For now, just check if auth exists
+	return auth != nil
+}
+
+// selectAuth selects an available auth for the model.
+func (p *OAuthProvider) selectAuth(model string) *coreauth.Auth {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	for _, auth := range p.auths {
+		if p.isAuthAvailable(auth, model) {
+			return auth
+		}
+	}
+	return nil
+}
+
+// Errors
+var (
+	ErrNoAvailableAuth = errors.New("no available OAuth auth for model")
+)