Update proxy to Zhanlu v1.4.2 provider flow

The 1.4.2 extension replaced the old signed/encrypted chat gateway with an
OpenAI-compatible aigateway. Align the proxy with the new flow:

- Use ecloud.10086.cn login/model base URLs, zhanlu_ide plugin headers and
  v1.4.2 plugin version
- Provision the model API key via SM2-signed get-or-create after v1/login
  profile fetch; store api_key/model_base_url/email in credentials
- Chat via Bearer apiKey against {modelBaseUrl}/chat/completions with plain
  OpenAI SSE passthrough; fetch /v1/models from the gateway model-info endpoint
- Force HTTP/1.1 upstream (gateway drops HTTP/2 ALPN negotiation with EOF)
- Drop obsolete AES body encryption, model name mapping and vscode headers

internal/sign/signer.go

@@ -46,7 +46,9 @@ func (s Signer) BuildOpURL(path string, creds auth.Credentials, baseURL string,
 	if err != nil {
 		return "", err
 	}
-	timestamp := now().Format("2006-01-02T15:04:05Z")
+	// The plugin formats Beijing time (UTC+8) with a Z suffix via
+	// new Date(now.getTime()+8*3600*1000) and getUTC* accessors.
+	timestamp := now().Add(8 * time.Hour).UTC().Format("2006-01-02T15:04:05Z")
 	query := "AccessKey=" + creds.AccessKey +
 		"&SignatureMethod=HmacSHA1" +
 		"&SignatureNonce=" + nonce() +