fix(grid): eliminate redundant debounce timer and duplicate validation in DataGridValuePanel #1431

Summary

In lib/features/workspace/data_grid_value_panel.dart (lines 96–105):

if (oldWidget.cellValue != widget.cellValue) {
  _debounceTimer?.cancel();
  _initialDetectedLanguage = _detectLanguage(widget.cellValue);
  _preserveCompact = _isCompact(widget.cellValue);
  _controller.text = _formatInitialValue(widget.cellValue);
  _validateContent();
}

When _controller.text = ... is invoked, it synchronously calls _onTextChanged, which immediately schedules a 150 ms _debounceTimer to run _validateContent(). Right after setting text, didUpdateWidget runs _validateContent() synchronously, and 150 ms later the timer runs _validateContent() a second time.

When rapidly switching selected rows or cells in the grid, this spawns redundant timers and double-parses JSON/XML content.

Proposed Solution

Suppress or cancel the debounce timer during programmatic cellValue synchronization in didUpdateWidget, or guard _onTextChanged against programmatic updates.

Acceptance Criteria

• Programmatic cell updates perform validation once without scheduling redundant debounce timers.
• User typing in the value panel retains the 150 ms debounce behavior.