fix: DH-22684: ui.text_field preserves user input while focused

[mofojed]

• Do not overwrite the input value with server-pushed propValue while the
  text field is focused; sync on focus->blur transition instead.
• Stop passing defaultValue as the controlled value when no value prop is
  provided.
• Add current input value to the serialized FocusEvent payload so on_focus
  / on_blur handlers can read what the user typed.
• Add e2e test covering the laggy-on_change focused-typing scenario, focus and change events.

[Copilot]

Pull request overview

This PR addresses DH-22684 by preventing ui.text_field from overwriting in-progress user typing while the field is focused, and by enriching focus/blur event payloads so server-side handlers can read the current input value.

Changes:

• Add focus-tracking + updated debounced value syncing so server-pushed value updates are deferred until blur.
• Extend serialized focus events to include the current input element value.
• Add Playwright e2e coverage (including a slow server on_change scenario) and a test app panel to exercise focus/change/blur payloads.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/utils.ts	Type-only import cleanup for Playwright types.
tests/ui_text_field.spec.ts	New e2e coverage for focused typing + focus/change/blur payload assertions.
tests/app.d/ui_text_field.py	Adds test panels used by the new e2e spec (slow on_change + event logging).
tests/app.d/tests.app	Registers the new test app panel module.
plugins/ui/src/js/src/elements/hooks/useTextInputProps.ts	Tracks focus state and passes it into debounced change handling.
plugins/ui/src/js/src/elements/hooks/useFocusEventCallback.ts	Serializes current target value into focus/blur events when available.
plugins/ui/src/js/src/elements/hooks/useDebouncedOnChange.ts	Skips server→UI value sync while focused; syncs on blur transition.
plugins/ui/src/deephaven/ui/components/types/events.py	Updates Python FocusEvent typing to include value.

Comments suppressed due to low confidence (1)

plugins/ui/src/js/src/elements/hooks/useTextInputProps.ts:51

• useTextInputProps now calls useDebouncedOnChange(propValue, ...) but still always returns a value prop. When propValue is omitted (uncontrolled usage with only defaultValue), the hook initializes value to undefined and then sets it to a string on first user edit, which causes an uncontrolled→controlled transition (React warning) and effectively reintroduces local control of the field. Consider branching: if propValue is undefined, omit value from the returned props (and avoid storing local value state), while still wiring onChange to call the debounced server callback; only provide a controlled value when propValue is defined.

  const [value, onChange] = useDebouncedOnChange(
    propValue,
    propOnChange,
    isFocused
  );

  return {
    defaultValue,
    value,
    onChange,

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