Initialize Window event handlers before impl construction (#4347)

[mvanhorn]

Closes #4347 (and the duplicate #4357).

Summary

When toga.Window.__init__ creates self._impl, some platform layers fire resize / focus callbacks before construction returns:

• Cocoa: windowDidResize_ fires when the requested size has to be clamped to fit the screen or the contained widget's natural size (the (128, 128) window with a Label(width=256) reproducer in the issue).
• WinForms .NET Framework 4.x: Activated fires during Form creation when the window becomes the active form (per documentapp sample: exception 'DocumentWindow' object has no attribute '_on_gain_focus' on Windows #4357).

Both callbacks dispatch through the interface property getters, which read self._on_resize / self._on_gain_focus / etc. directly. Those attributes are only assigned at the end of __init__ via the self.on_X = on_X lines, so the in-construction callback raises:

AttributeError: 'Window' object has no attribute '_on_resize'

Fix

Install wrapped no-op handlers (wrapped_handler(self, None)) for all six on_X attributes before constructing self._impl. The explicit self.on_X = on_X assignments at the end of __init__ still run and override these defaults with the user-supplied handlers, so behavior is unchanged for the normal flow.

This matches the maintainer's analysis on #4347 and #4357: "the on_gain_focus handler needs to exist before the window is created."

Tests

Adds test_window_handler_attrs_initialized_before_impl in core/tests/window/test_window.py that monkey-patches the dummy Window.__init__ to fire all six interface callbacks from inside its constructor. The test passes on this branch and fails on main with AttributeError: 'Window' object has no attribute '_on_resize' -- exactly the bug from the issue.

$ python -m pytest core/tests/window/ -q
109 passed

PR Checklist

• All new features have been tested
• All new features have been documented (changenote changes/4347.bugfix.md)
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the PR! I've cleaned up a couple of pieces in the implementation (details inline); but this was really helpful as a starting point (especially the test case).

[freakboy3742]

The wrapping can be done automatically by assigning None as the actual handler (and that also ensures that cleanup behaviour is installed correctly - e.g., in the case of on_close)

Suggested change

	self._on_close = wrapped_handler(self, None)
	self._on_gain_focus = wrapped_handler(self, None)
	self._on_lose_focus = wrapped_handler(self, None)
	self._on_show = wrapped_handler(self, None)
	self._on_hide = wrapped_handler(self, None)
	self._on_resize = wrapped_handler(self, None)
	self.on_close = None
	self.on_gain_focus = None
	self.on_lose_focus = None
	self.on_show = None
	self.on_hide = None
	self.on_resize = None

[freakboy3742]

A loop through a number of different test assertions is a likely indicator that a test should be parameterised. That way we'll get an independent failure signal if one of the handler is modified unexpectedly.

[freakboy3742]

We can be a bit more terse and user-focused here. The final release note will reference the ticket number, so someone wanting more detail can find out more; so we only indicate the nature of the problem at a high level in the release note.

Suggested change

	Window event handlers are now initialized to no-op defaults before the platform implementation is created, so resize and focus callbacks that fire during impl construction (Cocoa ``windowDidResize_``, WinForms .NET Framework 4.x ``Activated``) no longer raise ``AttributeError`` on ``_on_resize`` / ``_on_gain_focus`` / etc.
	Runtime errors caused by Window event handlers firing as part of the window initialization process have now been silenced.