fix: replace deprecated MediaType.parse() with toMediaType() extension

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android/app/src/main/java/com/tiangong/aiagent/data/remote/AuthInterceptor.kt

@@ -8,6 +8,7 @@ import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.launch
 import okhttp3.*
+import okhttp3.MediaType.Companion.toMediaType
 import java.io.IOException
 
 class AuthInterceptor(
@@ -89,7 +90,7 @@ class AuthInterceptor(
 
             val client = OkHttpClient()
             val body = okhttp3.RequestBody.create(
-                okhttp3.MediaType.parse("application/json")!!,
+                "application/json".toMediaType(),
                 """{"refresh_token":"$refreshToken"}"""
             )
             val refreshUrl = (cachedServerUrl ?: com.tiangong.aiagent.BuildConfig.BASE_URL).trimEnd('/') + "/api/v1/auth/refresh"