fix(editor): prevent length mismatch assertion error in QueryaHighlightController for scripts > 8KB #909

Summary

In QueryaHighlightController.buildTextSpan (lib/core/editor/querya_highlight_controller.dart, lines 70–82):
When editing text whose length exceeds kSyntaxHighlightIsolateThreshold (8192 characters), syntax highlighting is offloaded to an isolate with a 100ms debounce.

While waiting for the background isolate to complete:

if (_cachedSpan != null) {
  return TextSpan(style: style, children: [_cachedSpan!]);
}
return TextSpan(style: style, text: text);

If the user is actively typing or deleting characters, _cachedSpan holds the TextSpan from the previous text version (_cachedText), while text has changed.

Flutter's RenderEditable requires that span.toPlainText() == text. Returning a stale _cachedSpan with mismatched length causes Assertion failed: span.toPlainText() == text in debug mode, and corrupts cursor rendering and selection offsets in release mode.

Repro

1. Open a SQL script larger than 8 KB in Querya SQL Editor.
2. Type or delete characters in the editor.
3. Observe AssertionError: span.toPlainText() == text or cursor offset jumping while typing before the 100ms debounce finishes.

Scope

• In QueryaHighlightController.buildTextSpan, verify _cachedText == text before returning _cachedSpan.
• Fall back to plain TextSpan(style: style, text: text) while new highlight tokens are pending from the isolate.

Out of scope

• Grammar parsing logic in syntax_highlight package.