fix(grid): clear primary key columns when duplicating a row in staging buffer

[ZhuchkaTriplesix]

Summary

In VirtualResultGrid._handleDuplicateRow:

void _handleDuplicateRow(int row) {
  if (widget.stagingBuffer == null || row >= _sortedRows.length) return;
  final rowData = _sortedRows[row];
  widget.stagingBuffer!.addRow(List<String>.from(rowData));
}

When a user triggers "Duplicate Row" (via row context menu or keyboard shortcut), the row is cloned verbatim into the staging buffer, including the primary key value (e.g. id = 42).

When the user clicks Save, TableMutationEngine constructs an INSERT query containing the duplicate primary key:
INSERT INTO users (id, name, email) VALUES (42, 'Alice', ...);
This causes an immediate database error (duplicate key value violates unique constraint / PRIMARY KEY must be unique), failing the save operation.

Repro

1. Open Table Browser on a table with a primary key.
2. Right-click any row and select "Duplicate Row".
3. A new row appears at the bottom with all identical values, including the PK.
4. Click Save.
5. Save fails with a unique constraint violation error.

Scope

• When duplicating a row, identify primary key columns (using primaryKeys / columnMeta).
• Reset primary key cell values to empty string or default sentinel so the database can generate a fresh unique key.
• Alternatively, provide a duplicateRow(int index, {Set<String> primaryKeys}) helper on DataGridStagingBuffer that blanks PK columns automatically.
• Add tests in test/features/workspace/data_grid_staging_buffer_test.dart.

Out of scope

• Natural composite keys where the user is required to manually input distinct key components (the user can still edit the blank cell before saving).

[ZhuchkaTriplesix]

Resolved by #962 (already merged into dev; not auto-closed since this repo's default branch is main and PRs merge into dev).