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/apikey.go

@@ -0,0 +1,156 @@
+package providers
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"strings"
+	"sync"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/routing"
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+)
+
+// APIKeyProvider wraps API key configs as routing.Provider.
+type APIKeyProvider struct {
+	name     string
+	provider string // claude, gemini, codex, vertex
+	keys     []APIKeyEntry
+	mu       sync.RWMutex
+	client   HTTPClient
+}
+
+// APIKeyEntry represents a single API key configuration.
+type APIKeyEntry struct {
+	APIKey  string
+	BaseURL string
+	Models  []config.ClaudeModel // Using ClaudeModel as generic model alias
+}
+
+// HTTPClient interface for making HTTP requests.
+type HTTPClient interface {
+	Do(req *http.Request) (*http.Response, error)
+}
+
+// NewAPIKeyProvider creates a new API key provider.
+func NewAPIKeyProvider(name, provider string, client HTTPClient) *APIKeyProvider {
+	return &APIKeyProvider{
+		name:     name,
+		provider: provider,
+		keys:     make([]APIKeyEntry, 0),
+		client:   client,
+	}
+}
+
+// Name returns the provider name.
+func (p *APIKeyProvider) Name() string {
+	return p.name
+}
+
+// Type returns ProviderTypeAPIKey.
+func (p *APIKeyProvider) Type() routing.ProviderType {
+	return routing.ProviderTypeAPIKey
+}
+
+// SupportsModel checks if the model is supported by this provider.
+func (p *APIKeyProvider) SupportsModel(model string) bool {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	for _, key := range p.keys {
+		for _, m := range key.Models {
+			if strings.EqualFold(m.Alias, model) || strings.EqualFold(m.Name, model) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// Available always returns true for API keys (unless explicitly disabled).
+func (p *APIKeyProvider) Available(model string) bool {
+	return p.SupportsModel(model)
+}
+
+// Priority returns the priority (API key is lower priority than OAuth).
+func (p *APIKeyProvider) Priority() int {
+	return 20
+}
+
+// Execute sends the request using the API key.
+func (p *APIKeyProvider) Execute(ctx context.Context, model string, req executor.Request) (executor.Response, error) {
+	key := p.selectKey(model)
+	if key == nil {
+		return executor.Response{}, ErrNoMatchingAPIKey
+	}
+
+	// Resolve the actual model name from alias
+	actualModel := p.resolveModel(key, model)
+
+	// Execute via HTTP client
+	return p.executeHTTP(ctx, key, actualModel, req)
+}
+
+// ExecuteStream sends a streaming request.
+func (p *APIKeyProvider) ExecuteStream(ctx context.Context, model string, req executor.Request) (
+	<-chan executor.StreamChunk, error) {
+	key := p.selectKey(model)
+	if key == nil {
+		return nil, ErrNoMatchingAPIKey
+	}
+
+	actualModel := p.resolveModel(key, model)
+	return p.executeHTTPStream(ctx, key, actualModel, req)
+}
+
+// AddKey adds an API key entry.
+func (p *APIKeyProvider) AddKey(entry APIKeyEntry) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.keys = append(p.keys, entry)
+}
+
+// selectKey selects a key that supports the model.
+func (p *APIKeyProvider) selectKey(model string) *APIKeyEntry {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	for _, key := range p.keys {
+		for _, m := range key.Models {
+			if strings.EqualFold(m.Alias, model) || strings.EqualFold(m.Name, model) {
+				return &key
+			}
+		}
+	}
+	return nil
+}
+
+// resolveModel resolves alias to actual model name.
+func (p *APIKeyProvider) resolveModel(key *APIKeyEntry, requested string) string {
+	for _, m := range key.Models {
+		if strings.EqualFold(m.Alias, requested) {
+			return m.Name
+		}
+	}
+	return requested
+}
+
+// executeHTTP makes the HTTP request.
+func (p *APIKeyProvider) executeHTTP(ctx context.Context, key *APIKeyEntry, model string, req executor.Request) (executor.Response, error) {
+	// TODO: implement actual HTTP execution
+	// This is a placeholder - actual implementation would build HTTP request
+	return executor.Response{}, errors.New("not yet implemented")
+}
+
+// executeHTTPStream makes a streaming HTTP request.
+func (p *APIKeyProvider) executeHTTPStream(ctx context.Context, key *APIKeyEntry, model string, req executor.Request) (
+	<-chan executor.StreamChunk, error) {
+	// TODO: implement actual HTTP streaming
+	return nil, errors.New("not yet implemented")
+}
+
+// Errors
+var (
+	ErrNoMatchingAPIKey = errors.New("no API key supports the requested model")
+)