migrate all users to user goal check

[LawyZheng]

[!IMPORTANT] Removed feature flag for user goal checks in agent_step, updated related functions, and ensured page retrieval in BrowserState.

• Behavior:
  • Removed feature flag check for "CHECK_USER_GOAL_SUCCESS_EVERY_STEP" in agent_step() in agent.py, now always checks user goal after re-scraping.
  • Updated check_user_goal_complete() to return CompleteAction or None instead of a tuple.
  • Updated handle_complete_action() in handler.py to use check_user_goal_complete().
• Functions:
  • Renamed check_user_goal_success() to check_user_goal_complete() in agent.py.
  • Added must_get_working_page() to BrowserState in browser_factory.py to ensure a page is always returned.
• Imports:
  • Removed unused import extract_information_for_navigation_goal from agent.py.
  • Removed unused import BrowserState from handler.py.

^{This description was created by for 2964ba2113a9a2a626f5810255696c296d16fd10. It will automatically update as commits are pushed.}