fix: tear down area detector live view on close

[Osayi-ANL]

Stops the area detector’s live timers and PVA/CA monitors cleanly when the window closes, preventing callbacks from outliving the viewer.

Test plan:

• Open Area Detector.
• Start a live stream.
• Close Area Detector while the stream is live.
• Confirm no error appears in the terminal.

[pecomyint]

Please own and stop/join the PV-poller thread before clearing monitors, and run reader cleanup unconditionally. A close-during-poller test still registered an HKL monitor after close and triggered the EPICS thread teardown error.

[pecomyint]

Please update this branch after #172 lands: both PRs modify BaseWindow.closeEvent, and the resolved method must retain #172's save_layout() plus this PR's begin_close()/stop_child_timers() behavior

[Osayi-ANL]

Rebased onto main after #172. BaseWindow.closeEvent now runs save_layout(), then begin_close() and stop_child_timers().

Review points:

• The PV poller thread is owned (_pv_poller_thread) and joined with no timeout before any monitor is cleared; the sweep also checks is_closing between steps.
• Reader cleanup (ROI backup monitor + stop_channel_monitor()) runs unconditionally, each step guarded separately.
• Covered by test_teardown_waits_for_the_poller_before_clearing_monitors, test_poller_join_has_no_timeout and test_teardown_cleans_the_reader_even_when_the_pva_monitor_is_idle.