🐛 fix(external-sql): 修复外部 SQL 文件丢失后标签无法关闭

- 后端读取 SQL 文件失败时返回 file_not_found 结构化错误码
- 前端识别文件被删除或移动的场景，允许用户确认关闭标签
- 保留权限、网络盘异常等非缺失错误的关闭拦截，避免误丢草稿
- 补充前后端测试覆盖缺失文件识别与标签关闭提示
Close #566

frontend/src/utils/sqlFileTabDirty.test.ts

@@ -3,6 +3,8 @@ import { describe, expect, it } from 'vitest';
 import {
   getSQLFileTabPath,
   hasSQLFileTabUnsavedChanges,
+  isSQLFileMissingErrorMessage,
+  isSQLFileMissingReadResult,
   isSQLFileQueryTab,
   normalizeSQLFileReadContent,
 } from './sqlFileTabDirty';
@@ -34,4 +36,24 @@ describe('sqlFileTabDirty', () => {
       query: 'select 2;',
     } as any, 'select 1;')).toBe(true);
   });
+
+  it('detects missing SQL file read failures by structured error code', () => {
+    expect(isSQLFileMissingReadResult({
+      success: false,
+      message: '无法读取文件信息: stat /tmp/missing.sql: no such file or directory',
+      data: { errorCode: 'file_not_found', filePath: '/tmp/missing.sql' },
+    })).toBe(true);
+
+    expect(isSQLFileMissingReadResult({
+      success: false,
+      message: '无法读取文件信息: permission denied',
+      data: { filePath: '/tmp/report.sql' },
+    })).toBe(false);
+  });
+
+  it('keeps platform-specific missing file messages as a fallback', () => {
+    expect(isSQLFileMissingErrorMessage('GetFileAttributesEx C:\\Users\\me\\missing.sql: The system cannot find the file specified.')).toBe(true);
+    expect(isSQLFileMissingErrorMessage('stat /Users/me/missing.sql: no such file or directory')).toBe(true);
+    expect(isSQLFileMissingErrorMessage('无法读取文件信息: 权限不足')).toBe(false);
+  });
 });