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/claude/anthropic_auth_proxy_test.go

@@ -0,0 +1,33 @@
+package claude
+
+import (
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	"golang.org/x/net/proxy"
+)
+
+func TestNewClaudeAuthWithProxyURL_OverrideDirectTakesPrecedence(t *testing.T) {
+	cfg := &config.Config{SDKConfig: config.SDKConfig{ProxyURL: "socks5://proxy.example.com:1080"}}
+	auth := NewClaudeAuthWithProxyURL(cfg, "direct")
+
+	transport, ok := auth.httpClient.Transport.(*utlsRoundTripper)
+	if !ok || transport == nil {
+		t.Fatalf("expected utlsRoundTripper, got %T", auth.httpClient.Transport)
+	}
+	if transport.dialer != proxy.Direct {
+		t.Fatalf("expected proxy.Direct, got %T", transport.dialer)
+	}
+}
+
+func TestNewClaudeAuthWithProxyURL_OverrideProxyAppliedWithoutConfig(t *testing.T) {
+	auth := NewClaudeAuthWithProxyURL(nil, "socks5://proxy.example.com:1080")
+
+	transport, ok := auth.httpClient.Transport.(*utlsRoundTripper)
+	if !ok || transport == nil {
+		t.Fatalf("expected utlsRoundTripper, got %T", auth.httpClient.Transport)
+	}
+	if transport.dialer == proxy.Direct {
+		t.Fatalf("expected proxy dialer, got %T", transport.dialer)
+	}
+}