Record backend input events in the event hook

[tilladam]

What

Feeds real backend input (winit and Qt) through the window_event_hook so
that user-generated pointer, wheel and key events are observable to the hook,
each reported exactly once with its true dispatch result.

Previously the hook only saw events that were replayed through the public
Window::dispatch_event_with_result; input that arrived directly from the
backend and went straight into process_mouse_input / process_key_input
was invisible to it.

How

• WindowInner gains two #[doc(hidden)] helpers,
  process_mouse_input_and_notify and process_key_input_and_notify, which
  run the normal internal dispatch and then notify the hook with the event's
  public WindowEvent representation and its WindowEventDispatchResult.
• A shared notify_window_event helper does the context/hook lookup;
  dispatch_event_with_result is refactored to reuse it (no behavior change).
• The winit and Qt backends map each raw input event to its public
  WindowEvent and route it through the new helpers.
• PointerExited is reported as Accepted unconditionally, matching the
  existing teardown-event semantics in dispatch_event_with_result.
• In Qt, a drag-leave is deliberately kept off the recording path (via a new
  drag_leave_event) so it is not mis-recorded as a pointer exit.

Tests

Adds dispatch_result_tests coverage in the testing backend asserting that
the new backend helpers notify the hook exactly once with the correct result
for pointer, key and pointer-exit events.

Notes / follow-ups

• Depends on / pairs with the one-line winit fix "winit: propagate the key
  auto-repeat flag to KeyEvent" (separate PR). This PR is independent and
  green on its own — it reads winit's event.repeat to emit
  KeyPressRepeated for recording — but the two are best merged together,
  the repeat fix first.
• Touch and gesture input (winit Touch, Qt pinch/rotation, multi-touch) is
  not recorded: the public platform::WindowEvent has no variant for it.
  Left as a follow-up.
• Scroll phase does not round-trip through the public PointerScrolled
  variant, which has no phase field.