[codex] expand task form coverage

[abhay-byte]

What changed

• Added recurrence-focused coverage for task creation and task form save behavior.
• Added widget tests for the New Task form controls: colors, tags, recurrence, reminders, goals, and graphic selection.
• Added supporting test hooks on the task form screen so the widget tests can target controls reliably.

Validation

• flutter test test/domain/task_form_notifier_test.dart test/widgets/task_form_screen_test.dart
• flutter test

Notes

• This is a test-focused change set; no production behavior changes beyond lightweight test keys on the form screen.