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/kimi/kimi_proxy_test.go

@@ -0,0 +1,42 @@
+package kimi
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+)
+
+func TestNewDeviceFlowClientWithDeviceIDAndProxyURL_OverrideDirectDisablesProxy(t *testing.T) {
+	cfg := &config.Config{SDKConfig: config.SDKConfig{ProxyURL: "http://proxy.example.com:8080"}}
+	client := NewDeviceFlowClientWithDeviceIDAndProxyURL(cfg, "device-1", "direct")
+
+	transport, ok := client.httpClient.Transport.(*http.Transport)
+	if !ok || transport == nil {
+		t.Fatalf("expected http.Transport, got %T", client.httpClient.Transport)
+	}
+	if transport.Proxy != nil {
+		t.Fatal("expected direct transport to disable proxy function")
+	}
+}
+
+func TestNewDeviceFlowClientWithDeviceIDAndProxyURL_OverrideProxyTakesPrecedence(t *testing.T) {
+	cfg := &config.Config{SDKConfig: config.SDKConfig{ProxyURL: "http://global.example.com:8080"}}
+	client := NewDeviceFlowClientWithDeviceIDAndProxyURL(cfg, "device-1", "http://override.example.com:8081")
+
+	transport, ok := client.httpClient.Transport.(*http.Transport)
+	if !ok || transport == nil {
+		t.Fatalf("expected http.Transport, got %T", client.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)
+	}
+}