feat(auth): add proxy URL override support to auth constructors and executors

- Introduced `WithProxyURL` variants for `CodexAuth`, `ClaudeAuth`, `IFlowAuth`, and `DeviceFlowClient`.
- Updated executors to use proxy-aware constructors for improved configurability.
- Added unit tests to validate proxy override precedence and functionality.

Closes: #2823

internal/auth/iflow/iflow_auth.go

@@ -48,8 +48,23 @@ type IFlowAuth struct {
 
 // NewIFlowAuth constructs a new IFlowAuth with proxy-aware transport.
 func NewIFlowAuth(cfg *config.Config) *IFlowAuth {
+	return NewIFlowAuthWithProxyURL(cfg, "")
+}
+
+// NewIFlowAuthWithProxyURL constructs a new IFlowAuth with a proxy override.
+// proxyURL takes precedence over cfg.ProxyURL when non-empty.
+func NewIFlowAuthWithProxyURL(cfg *config.Config, proxyURL string) *IFlowAuth {
 	client := &http.Client{Timeout: 30 * time.Second}
-	return &IFlowAuth{httpClient: util.SetProxy(&cfg.SDKConfig, client)}
+	effectiveProxyURL := strings.TrimSpace(proxyURL)
+	var sdkCfg config.SDKConfig
+	if cfg != nil {
+		sdkCfg = cfg.SDKConfig
+		if effectiveProxyURL == "" {
+			effectiveProxyURL = strings.TrimSpace(cfg.ProxyURL)
+		}
+	}
+	sdkCfg.ProxyURL = effectiveProxyURL
+	return &IFlowAuth{httpClient: util.SetProxy(&sdkCfg, client)}
 }
 
 // AuthorizationURL builds the authorization URL and matching redirect URI.

internal/auth/iflow/iflow_auth_test.go

@@ -0,0 +1,42 @@
+package iflow
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+)
+
+func TestNewIFlowAuthWithProxyURL_OverrideDirectDisablesProxy(t *testing.T) {
+	cfg := &config.Config{SDKConfig: config.SDKConfig{ProxyURL: "http://proxy.example.com:8080"}}
+	auth := NewIFlowAuthWithProxyURL(cfg, "direct")
+
+	transport, ok := auth.httpClient.Transport.(*http.Transport)
+	if !ok || transport == nil {
+		t.Fatalf("expected http.Transport, got %T", auth.httpClient.Transport)
+	}
+	if transport.Proxy != nil {
+		t.Fatal("expected direct transport to disable proxy function")
+	}
+}
+
+func TestNewIFlowAuthWithProxyURL_OverrideProxyTakesPrecedence(t *testing.T) {
+	cfg := &config.Config{SDKConfig: config.SDKConfig{ProxyURL: "http://global.example.com:8080"}}
+	auth := NewIFlowAuthWithProxyURL(cfg, "http://override.example.com:8081")
+
+	transport, ok := auth.httpClient.Transport.(*http.Transport)
+	if !ok || transport == nil {
+		t.Fatalf("expected http.Transport, got %T", auth.httpClient.Transport)
+	}
+	req, errReq := http.NewRequest(http.MethodGet, "https://example.com", nil)
+	if errReq != nil {
+		t.Fatalf("new request: %v", errReq)
+	}
+	proxyURL, errProxy := transport.Proxy(req)
+	if errProxy != nil {
+		t.Fatalf("proxy func: %v", errProxy)
+	}
+	if proxyURL == nil || proxyURL.String() != "http://override.example.com:8081" {
+		t.Fatalf("proxy URL = %v, want http://override.example.com:8081", proxyURL)
+	}
+}