perf(mysql): add StringInternPool to MySqlSqlExecutionDelegate row conversion #1356

Summary

In lib/features/mysql/mysql_sql_workspace.dart (lines 110–121):

final outRows = [
  for (final row in taken.items)
    List.generate(
      row.numOfColumns,
      (i) => mysqlResultCellToDisplayString(row.colAt(i), column: col),
    ),
];

mysqlResultCellToDisplayString in lib/core/database/mysql_result_cells.dart supports an optional StringInternPool? pool parameter, but MySqlSqlExecutionDelegate never passes a pool.
Unlike PostgreSQL (which uses StringInternPool via convertPostgresResultRowsToStringsAdaptive), MySQL allocates fresh string instances for every single cell. For large result sets with repetitive status, boolean, foreign key, or category values, this creates tens of thousands of duplicate heap string allocations.

Proposed Solution

1. Instantiate and pass a StringInternPool during MySQL row generation in MySqlSqlExecutionDelegate.executeQuery.
2. Yield to the event loop every kResultStringConvertYieldEvery rows during conversion to prevent UI frame hitching.

Acceptance Criteria

• Identical string cell values across rows in MySQL share interned references.
• Memory usage during MySQL query execution is measurably reduced.