test: 添加 background 单元测试与边界情况处理

- 新增 entrypoints/__tests__/background.test.ts (12个测试用例)
- 超长文本截断限制 (>10000字符)
- Base64 内联图片拦截并返回错误提示
- 更新 parseContextMenuClick 返回 ParseResult 格式
- 更新测试匹配新的返回格式
- lint/typecheck/test 全部通过 (579个测试)

entrypoints/background.ts

@@ -10,9 +10,15 @@ export default defineBackground(() => {
 
   browser.contextMenus.onClicked.addListener(async (info, _tab) => {
     const result = parseContextMenuClick(info.menuItemId as string, info);
-    if (!result) return;
 
-    const { featureKey, payload } = result;
+    if (!result.success || !result.data) {
+      if (result.error) {
+        console.warn('[Context Menu]', result.error);
+      }
+      return;
+    }
+
+    const { featureKey, payload } = result.data;
 
     try {
       const sidePanelState = await browser.storage.local.get('sidePanelOpen');