🔧 chore(sdk-test): update certificate field configuration

- set default certificate value from environment variable VITE_AGORA_APP_CERTIFICATE
- change certificate input from password type to regular text input for better visibility

src/pages/sdk-test/index.tsx

@@ -246,7 +246,7 @@ const SdkTestPage = () => {
               layout="vertical"
               initialValues={{
                 appId: import.meta.env.VITE_AGORA_APP_ID || "",
-                certificate: "",
+                certificate: import.meta.env.VITE_AGORA_APP_CERTIFICATE || "",
                 channel: `test-channel-${Date.now()}`,
                 userName: `TestUser-${Date.now()}`,
               }}
@@ -264,7 +264,7 @@ const SdkTestPage = () => {
                 name="certificate"
                 rules={[{ required: true, message: "请输入 Certificate" }]}
               >
-                <Input.Password placeholder="请输入 Agora Certificate" />
+                <Input placeholder="请输入 Agora Certificate" />
               </Form.Item>
 
               <Form.Item