Code cleanup, hardening, and test coverage improvements

[sreckoskocilic]

Summary

Hardening (assert → raise, guards, logging)

• model/model.py — assert → raise RuntimeError/ValueError; Event-based shutdown flag; Lock for _files dict; _snapshot_files() for safe cross-thread iteration; early return in _record_files when shutdown; _on_empty_rows_inited runs callback via @run_in_main_thread; _commit_sort_updates_and_sort moves sort value writes to main thread
• model/table.py — Lock guards on Rows.__len__, __getitem__, __iter__, index; assert → raise RuntimeError
• model/worker.py — task_done() on shutdown sentinel; join() guarded by is_alive() check
• onboarding/tutorial.py — 3 assert → raise ValueError with context messages
• usage_helper.py — assert events → early return False on empty events list
• single_row_mode.py — null guard before remove_delegate
• quicksearch.py — guard against row_height <= 0
• onboarding/__init__.py — unclosed </ul> tag fix

Silent failures → logging

• metrics.py — bare pass in track()/update_user() → _LOG.debug() with exc_info; SENTINEL-based shutdown for AsynchronousMetrics thread
• plugins/config.py — inline comments explaining swallowed exceptions → _LOG.debug()/_LOG.warning() with exc_info
• plugins/error.py — docstring for report() exc parameter

Bug fixes

• controller.py — file_path → file_url parameter rename for consistency
• cursor_movement.py — toggle_selection now passed through in move_cursor_page_up/page_down
• util/path.py — normalize handles chained .. segments and /.. at root
• core/__init__.py — numeric sort padding widened from 6 to 20 digits (supports numbers up to 10^20)
• plugins/config.py — type() == → isinstance() for proper subclass handling

Cleanup

• controller.py — removed assert on QContextMenuEvent.reason()
• explorer_properties.py — try/finally ensures DestroyMenu(hMenu) cleanup on Windows
• key_event.py — "OS X" → "macOS"; updated Qt doc URL to archived 5.15 docs

New tests (39 tests added)

• test_worker.py (new) — WorkerTest (6 tests): priority ordering, shutdown, submit-after-shutdown, exception resilience. WorkItemTest (4 tests): shutdown detection, ordering, exception capture
• test_error.py (new) — FormatTracebackTest (3), WalkTbWithFilteringTest (3), TracebackExceptionWithTbFilterTest (5), PluginErrorHandlerTest (4): traceback filtering, chained exceptions, pending error queuing
• test_resize_cols_to_contents.py — 2 new ResizeColumnTest methods (shrink width cap), DistributeEvenlyTest (4), DistributeExponentiallyTest (4)
• test_model.py — ModelFilesCopyTest: concurrent dict.copy() safety under mutation
• test_plugin.py — ReportExceptions tests: exception_stored, exclude_must_be_set
• test_path.py — normalize tests for chained .. and /.. at root
• test_quicksearch_matchers.py — ContainsCharsTest (6) and ContainsSubstringTest (4)

Test plan

• Run full test suite: all 246 tests pass, 0 failures
• Navigate directories with deep paths containing .. segments
• Resize columns — verify no overflow, selection toggle works with page up/down
• Verify metrics logging appears in debug output instead of silent failures