fix(editor): prevent duplicate onChanged events, cursor false positives, and missing default highlighting in QueryaCodeEditor #1430

Summary

In lib/core/editor/querya_code_editor.dart:

1. Duplicate onChanged invocations:
   In build(), onChanged: widget.onChanged is passed to TextField. At the same time, _attachListener() binds _onTextChanged to _effectiveController, which also executes widget.onChanged?.call(_effectiveController.text). This causes onChanged to fire twice per keystroke.
2. False positive onChanged on cursor moves / selection:
   _effectiveController.addListener(_onTextChanged) triggers on any change to TextEditingValue, including selection and cursor position. Without tracking _lastText, simply clicking or moving arrow keys invokes widget.onChanged.
3. Missing syntax highlighting when controller == null:
   enableHighlighting is declared with doc When true and [language] is SQL/JSON, uses [syntax_highlight] if initialized, but _initControllerIfNeeded() always creates a plain material.TextEditingController() without highlighters. enableHighlighting is never read.
4. didUpdateWidget ignores language/highlight changes:
   Only oldWidget.controller != widget.controller is checked. Toggling language does not reconfigure highlighters or controller.

Acceptance Criteria

• onChanged is called exactly once per text modification, and never on cursor/selection changes.
• When controller == null and enableHighlighting == true, QueryaCodeEditor creates a QueryaHighlightController via SyntaxHighlightService.
• didUpdateWidget reacts to changes in widget.language and widget.enableHighlighting.
• Unit tests cover duplicate prevention, cursor-movement safety, and default syntax highlighting.