perf(editor): eliminate full-string comparisons and redundant listeners in QueryaCodeEditor (#1352)

[ZhuchkaTriplesix]

Closes #1352

Summary

• Avoid attaching text change listener on the controller in QueryaCodeEditor when onChanged is null.
• Optimize _onTextChanged to use reference identity identical(currentText, _lastText) ((1)$) on cursor / selection changes without full-string comparison.
• Use length differential currentText.length != _lastText.length ((1)$) on typing/deletions, eliminating repeated multi-kilobyte string comparison passes across entire documents.
• Add dynamic listener re-attachment on onChanged updates in didUpdateWidget.
• Add test coverage for listener omission and same-length text replacement.