🐛 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

internal/app/methods_file.go

@@ -30,6 +30,8 @@ import (
 const minExportQueryTimeout = 5 * time.Minute
 const minClickHouseExportQueryTimeout = 2 * time.Hour
 const maxSQLFileSizeBytes int64 = 50 * 1024 * 1024
+
+const sqlFileErrorCodeNotFound = "file_not_found"
 const sqlFileBatchMaxStatements = 1000
 const sqlFileBatchMaxBytes = 4 * 1024 * 1024
 const sqlFileProgressStatementInterval = 100
@@ -321,7 +323,11 @@ func readSQLFileByPath(filePath string) connection.QueryResult {
 
 	fi, err := os.Stat(selection)
 	if err != nil {
-		return connection.QueryResult{Success: false, Message: fmt.Sprintf("无法读取文件信息: %v", err)}
+		data := map[string]interface{}{"filePath": selection}
+		if os.IsNotExist(err) {
+			data["errorCode"] = sqlFileErrorCodeNotFound
+		}
+		return connection.QueryResult{Success: false, Message: fmt.Sprintf("无法读取文件信息: %v", err), Data: data}
 	}
 	if fi.IsDir() {
 		return connection.QueryResult{Success: false, Message: "所选路径不是 SQL 文件"}

internal/app/methods_file_sql_directory_test.go

@@ -302,6 +302,26 @@ func TestReadSQLFileByPathReturnsLargeFileMetadata(t *testing.T) {
 	}
 }
 
+func TestReadSQLFileByPathMarksMissingFile(t *testing.T) {
+	filePath := filepath.Join(t.TempDir(), "missing.sql")
+
+	result := readSQLFileByPath(filePath)
+	if result.Success {
+		t.Fatalf("expected missing sql file read to fail, got %#v", result)
+	}
+
+	data, ok := result.Data.(map[string]interface{})
+	if !ok {
+		t.Fatalf("expected error metadata map, got %#v", result.Data)
+	}
+	if data["errorCode"] != sqlFileErrorCodeNotFound {
+		t.Fatalf("expected file_not_found error code, got %#v", data["errorCode"])
+	}
+	if data["filePath"] != filePath {
+		t.Fatalf("expected filePath %q, got %#v", filePath, data["filePath"])
+	}
+}
+
 func TestReadSQLFileWithMetadataByPathReturnsSmallFileContentAndPath(t *testing.T) {
 	filePath := filepath.Join(t.TempDir(), "report.sql")
 	if err := os.WriteFile(filePath, []byte("select 1;"), 0o644); err != nil {