validateAsync abstract method
Validates this control and any child controls asynchronously, updating errors.
Runs synchronous validators first. If synchronous validation fails, skips
asynchronous network calls and returns false immediately.
When debounce is true, delays validation by the control's configured debounce duration.
Overlapping runs are safely sequenced: results from stale runs are discarded.
Returns true if all synchronous and asynchronous validators pass; false otherwise.
Implementation
Future<bool> validateAsync({bool debounce = false});