Fixed: #2034

feat(proxy): centralize proxy handling with `proxyutil` package and enhance test coverage

- Added `proxyutil` package to simplify proxy handling across the codebase.
- Refactored various components (`executor`, `cliproxy`, `auth`, etc.) to use `proxyutil` for consistent and reusable proxy logic.
- Introduced support for "direct" proxy mode to explicitly bypass all proxies.
- Updated tests to validate proxy behavior (e.g., `direct`, HTTP/HTTPS, and SOCKS5).
- Enhanced YAML configuration documentation for proxy options.

sdk/cliproxy/rtprovider_test.go

@@ -0,0 +1,22 @@
+package cliproxy
+
+import (
+	"net/http"
+	"testing"
+
+	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
+)
+
+func TestRoundTripperForDirectBypassesProxy(t *testing.T) {
+	t.Parallel()
+
+	provider := newDefaultRoundTripperProvider()
+	rt := provider.RoundTripperFor(&coreauth.Auth{ProxyURL: "direct"})
+	transport, ok := rt.(*http.Transport)
+	if !ok {
+		t.Fatalf("transport type = %T, want *http.Transport", rt)
+	}
+	if transport.Proxy != nil {
+		t.Fatal("expected direct transport to disable proxy function")
+	}
+}