fix(proxy): log error when upstream TLS config creation fails

Previously the error was silently swallowed, causing the proxy to
fall back to default TLS settings (no custom CA, no mTLS, no SNI)
without any indication. Now the error is logged at ERROR level.

internal/proxy/proxy.go

@@ -337,7 +337,9 @@ func createHostClient(targetURL string, timeout config.ProxyTimeout, transportCf
 	// 上游 SSL 配置（使用原生 TLSConfig）
 	if sslCfg != nil && sslCfg.Enabled && isTLS {
 		tlsCfg, err := CreateTLSConfig(sslCfg, extractHostFromURL(targetURL))
-		if err == nil {
+		if err != nil {
+			logging.Error().Err(err).Str("target", targetURL).Msg("Failed to create upstream TLS config")
+		} else {
 			client.TLSConfig = tlsCfg
 		}
 	}