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/auth/aes.go

@@ -33,17 +33,16 @@ func DecryptCredential(ciphertextBase64, key string) (string, error) {
 	return string(plain), nil
 }
 
-func EncryptCredential(plaintext, key string) (string, error) {
-	block, err := aes.NewCipher(repeatKey(key, aes.BlockSize))
-	if err != nil {
-		return "", err
+// DecryptCredentialOrRaw mirrors the plugin's z4A: try AES-ECB decrypt with the
+// given key, falling back to the raw value when the field is plaintext.
+func DecryptCredentialOrRaw(value, key string) string {
+	if value == "" {
+		return ""
 	}
-	plain := padPKCS7([]byte(plaintext), aes.BlockSize)
-	out := make([]byte, len(plain))
-	for start := 0; start < len(plain); start += aes.BlockSize {
-		block.Encrypt(out[start:start+aes.BlockSize], plain[start:start+aes.BlockSize])
+	if plain, err := DecryptCredential(value, key); err == nil && plain != "" {
+		return plain
 	}
-	return base64.StdEncoding.EncodeToString(out), nil
+	return value
 }
 
 func repeatKey(key string, size int) []byte {
@@ -73,13 +72,3 @@ func unpadPKCS7(in []byte, blockSize int) ([]byte, error) {
 	}
 	return in[:len(in)-pad], nil
 }
-
-func padPKCS7(in []byte, blockSize int) []byte {
-	pad := blockSize - len(in)%blockSize
-	out := make([]byte, len(in)+pad)
-	copy(out, in)
-	for i := len(in); i < len(out); i++ {
-		out[i] = byte(pad)
-	}
-	return out
-}