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/sm2.go

@@ -0,0 +1,36 @@
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+	"strings"
+
+	"github.com/emmansun/gmsm/sm2"
+	"github.com/emmansun/gmsm/sm3"
+)
+
+// SignSM2Authorization signs `message` with the SM2 private key in hex form
+// (mirroring the Zhanlu plugin: SM3 digest signed with hash:false, der:false,
+// output as 64-byte r||s hex).
+func SignSM2Authorization(privateKeyHex, message string) (string, error) {
+	keyHex := strings.TrimPrefix(strings.TrimSpace(privateKeyHex), "0x")
+	keyBytes, err := hex.DecodeString(keyHex)
+	if err != nil {
+		return "", err
+	}
+	priv, err := sm2.NewPrivateKey(keyBytes)
+	if err != nil {
+		return "", err
+	}
+	digest := sm3.Sum([]byte(message))
+	r, s, err := sm2.Sign(rand.Reader, &priv.PrivateKey, digest[:])
+	if err != nil {
+		return "", err
+	}
+	rb := r.FillBytes(make([]byte, 32))
+	sb := s.FillBytes(make([]byte, 32))
+	return hex.EncodeToString(append(rb, sb...)), nil
+}
+
+var errEmptySM2Key = errors.New("SM2 private key is required")