fix(core): resolve latent bugs in grid filtering, MySQL escaping, export formatters, and staging buffer disposal

[ZhuchkaTriplesix]

Closes #698

Summary of Changes

1. Row Filtering Bypassed in Grid with StagingBuffer & Mutation Index Collision:

   • Added rowIndicesMapping to VirtualResultGrid.
   • In ResultsTab, cached filtered indices and passed them to VirtualResultGrid.
   • VirtualResultGrid renders rows when rowIndicesMapping is present, while mapping visual sorted rows back to underlying buffer row indices via _toModelRowIndex.
   • Fixed _openInspector, toggleDeleteRow, and revertRow shortcut handlers to route through _toModelRowIndex.

2. SQL Syntax Error / Escape Issue with Backslashes in MySQL DML Generation:

   • In TableMutationEngine, strings for SqlDialect.mysql now escape backslashes \ with \\ before escaping quotes.

3. Leading Zeros Stripping in JSON Export:

   • In ResultGridSelection.toJson, strings with leading zeros (e.g. ^0\d+$) are preserved as strings without being coerced to numbers.

4. TSV Export / Copy Delimiter Corruption on Newlines & Tabs:

   • In ResultGridSelection.toTsv, values containing \t, \n, \r, or " are now escaped with enclosing quotes and double quotes per standard TSV escaping.

5. StagingBuffer Memory Leak on Query Tab Close:

   • In SqlQueryTabSession.dispose, explicitly calls stagingBuffer?.dispose().

Verification

• flutter analyze: 0 issues found.
• All unit, widget, and workspace tests passing.