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/codex/openai_auth.go

@@ -37,8 +37,23 @@ type CodexAuth struct {
 // NewCodexAuth creates a new CodexAuth service instance.
 // It initializes an HTTP client with proxy settings from the provided configuration.
 func NewCodexAuth(cfg *config.Config) *CodexAuth {
+	return NewCodexAuthWithProxyURL(cfg, "")
+}
+
+// NewCodexAuthWithProxyURL creates a new CodexAuth service instance.
+// proxyURL takes precedence over cfg.ProxyURL when non-empty.
+func NewCodexAuthWithProxyURL(cfg *config.Config, proxyURL string) *CodexAuth {
+	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 &CodexAuth{
-		httpClient: util.SetProxy(&cfg.SDKConfig, &http.Client{}),
+		httpClient: util.SetProxy(&sdkCfg, &http.Client{}),
 	}
 }