fix(LLM 代理): Chat Completions 透传类型化错误，保留权限码

- resolve_llm_router_client 返回 Result<_, AppError> 而非 String（评审第 17 条）
- load_owner_llm_catalog 的 503 + MODEL_ACCESS_UNAVAILABLE 不再被拍平成「模型目录暂不可用」
- resolve_requested_for 错误改走 agc_model_resolve_error，与 proxy_llm_responses / proxy_llm_messages 契约一致
- LlmConfig / LlmClient 构造失败按 500 返回；调用方直接 llm_error_response 透传

server-rs/crates/api-server/src/llm/mod.rs

@@ -187,12 +187,7 @@ pub async fn proxy_llm_chat_completions(
     let (llm_client, key_id) =
         match resolve_llm_router_client(&state, authenticated.claims().user_id()).await {
             Ok((client, key_id)) => (client, key_id),
-            Err(error) => {
-                return Err(llm_error_response(
-                    &request_context,
-                    AppError::from_status(StatusCode::SERVICE_UNAVAILABLE).with_message(error),
-                ));
-            }
+            Err(error) => return Err(llm_error_response(&request_context, error)),
         };
     prepare_llm_router_billing(&state, authenticated.claims().user_id())
         .await
@@ -1518,16 +1513,20 @@ fn is_responses_terminal_sse_event(event: &str) -> bool {
 async fn resolve_llm_router_client(
     state: &AppState,
     owner_user_id: &str,
-) -> Result<(platform_llm::LlmClient, String), String> {
-    let (base_url, api_key, key_id) = resolve_llm_router_credentials(state, owner_user_id).await?;
-    let catalog = load_owner_llm_catalog(state, owner_user_id)
+) -> Result<(platform_llm::LlmClient, String), AppError> {
+    let (base_url, api_key, key_id) = resolve_llm_router_credentials(state, owner_user_id)
         .await
-        .map_err(|_| "模型目录暂不可用".to_string())?;
+        .map_err(|message| {
+            AppError::from_status(StatusCode::SERVICE_UNAVAILABLE).with_message(message)
+        })?;
+    // 目录读失败与会员权限读失败（503 + MODEL_ACCESS_UNAVAILABLE）都保留各自的类型化错误，
+    // 不再拍平成一句中文，与 `proxy_llm_responses` / `proxy_llm_messages` 的契约一致。
+    let catalog = load_owner_llm_catalog(state, owner_user_id).await?;
     // Chat Completions 的 `model` 恒为账号该档的默认模型，不接收客户端选择。
     let model = catalog
         .catalog
         .resolve_requested_for(None, catalog.access)
-        .map_err(|error| error.message().to_string())?;
+        .map_err(agc_model_resolve_error)?;
     let config = platform_llm::LlmConfig::new(
         platform_llm::LlmProvider::OpenAiCompatible,
         base_url.to_string(),
@@ -1537,10 +1536,16 @@ async fn resolve_llm_router_client(
         state.config.llm_max_retries,
         state.config.llm_retry_backoff_ms,
     )
-    .map_err(|error| error.to_string())?;
+    .map_err(|error| {
+        AppError::from_status(StatusCode::INTERNAL_SERVER_ERROR)
+            .with_message(format!("LLM Router 客户端配置非法：{error}"))
+    })?;
     platform_llm::LlmClient::new(config)
         .map(|client| (client, key_id))
-        .map_err(|error| error.to_string())
+        .map_err(|error| {
+            AppError::from_status(StatusCode::INTERNAL_SERVER_ERROR)
+                .with_message(format!("创建 LLM Router 客户端失败：{error}"))
+        })
 }
 
 async fn resolve_llm_router_credentials(