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/executor.go

@@ -0,0 +1,111 @@
+package routing
+
+import (
+	"context"
+	"errors"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
+	log "github.com/sirupsen/logrus"
+)
+
+// Executor handles request execution with fallback support.
+type Executor struct {
+	router *Router
+}
+
+// NewExecutor creates a new executor with the given router.
+func NewExecutor(router *Router) *Executor {
+	return &Executor{router: router}
+}
+
+// Execute sends the request through the routing decision.
+func (e *Executor) Execute(ctx context.Context, req executor.Request) (executor.Response, error) {
+	decision := e.router.Resolve(req.Model)
+	
+	log.Debugf("routing: %s -> %s (%d candidates)", 
+		decision.RequestedModel, 
+		decision.ResolvedModel, 
+		len(decision.Candidates))
+
+	var lastErr error
+	tried := make(map[string]struct{})
+
+	for i, candidate := range decision.Candidates {
+		key := candidate.Provider.Name() + "/" + candidate.Model
+		if _, ok := tried[key]; ok {
+			continue
+		}
+		tried[key] = struct{}{}
+
+		log.Debugf("routing: trying candidate %d/%d: %s with model %s",
+			i+1, len(decision.Candidates), candidate.Provider.Name(), candidate.Model)
+
+		req.Model = candidate.Model
+		resp, err := candidate.Provider.Execute(ctx, candidate.Model, req)
+		if err == nil {
+			return resp, nil
+		}
+
+		lastErr = err
+		log.Debugf("routing: candidate failed: %v", err)
+
+		// Check if it's a fatal error (not retryable)
+		if isFatalError(err) {
+			break
+		}
+	}
+
+	if lastErr != nil {
+		return executor.Response{}, lastErr
+	}
+	return executor.Response{}, errors.New("no available providers")
+}
+
+// ExecuteStream sends a streaming request through the routing decision.
+func (e *Executor) ExecuteStream(ctx context.Context, req executor.Request) (<-chan executor.StreamChunk, error) {
+	decision := e.router.Resolve(req.Model)
+
+	log.Debugf("routing stream: %s -> %s (%d candidates)",
+		decision.RequestedModel,
+		decision.ResolvedModel,
+		len(decision.Candidates))
+
+	var lastErr error
+	tried := make(map[string]struct{})
+
+	for i, candidate := range decision.Candidates {
+		key := candidate.Provider.Name() + "/" + candidate.Model
+		if _, ok := tried[key]; ok {
+			continue
+		}
+		tried[key] = struct{}{}
+
+		log.Debugf("routing stream: trying candidate %d/%d: %s with model %s",
+			i+1, len(decision.Candidates), candidate.Provider.Name(), candidate.Model)
+
+		req.Model = candidate.Model
+		chunks, err := candidate.Provider.ExecuteStream(ctx, candidate.Model, req)
+		if err == nil {
+			return chunks, nil
+		}
+
+		lastErr = err
+		log.Debugf("routing stream: candidate failed: %v", err)
+
+		if isFatalError(err) {
+			break
+		}
+	}
+
+	if lastErr != nil {
+		return nil, lastErr
+	}
+	return nil, errors.New("no available providers")
+}
+
+// isFatalError returns true if the error is not retryable.
+func isFatalError(err error) bool {
+	// TODO: implement based on error type
+	// For now, all errors are retryable
+	return false
+}