fix(input): fix AccessKeyHandler when no descendant control has focus

[AutumnYuanc]

What does the pull request do?

Fixes AccessKeyHandler so that Alt/mnemonic access keys work correctly even when no
descendant control has claimed keyboard focus yet — most notably right after a Window
is opened, before the user has pressed Tab or clicked anything.

What is the current behavior?

AccessKeyHandler.OnPreviewKeyDown / OnKeyDown guard their logic behind
IsFocusWithinOwner(_owner), which re-queries KeyboardDevice.Instance.FocusedElement
directly:

• Right after startup, KeyboardDevice.Instance.FocusedElement is null (nothing has
  claimed focus yet), so the pattern match focusedElement is not InputElement fails
  and IsFocusWithinOwner returns false immediately.
• Even once something does assign the owner/TopLevel itself as the focused element,
  Visual.IsVisualAncestorOf is a strict, non-reflexive ancestor check
  (x.IsVisualAncestorOf(x) is always false), so IsFocusWithinOwner still returns
  false when FocusedElement == owner.

In both cases OnPreviewKeyDown/OnKeyDown return early, so pressing Alt does nothing:
no access key underlines, no menu opening, no F10.

Meanwhile KeyboardDevice.ProcessRawEvent already resolves the effective focused
element as FocusedElement ?? e.Root.FocusRoot and uses that both as the routing target
and as KeyEventArgs.Source — so the event does correctly reach AccessKeyHandler's
handlers on _owner, it's only the internal guard that re-derives a different (and
incomplete) answer from scratch.

What is the updated/expected behavior with this PR?

Access keys (Alt + underline display, Alt+letter, F10/menu opening) work immediately
after a window opens, without requiring a prior Tab press or mouse click. Tests have
been added.

How was the solution implemented (if it's not obvious)?

IsFocusWithinOwner now takes the routed event's Source (already resolved by
KeyboardDevice.ProcessRawEvent using the same FocusedElement ?? e.Root.FocusRoot
fallback) instead of independently re-reading KeyboardDevice.Instance.FocusedElement.
This avoids duplicating focus-resolution logic and fixes both underlying issues at once:

• Source is never null for a real key event, so the "nothing focused yet" case is
  handled correctly.
• The check now also accepts ReferenceEquals(source, owner), fixing the
  IsVisualAncestorOf non-reflexivity issue for the case where the owner itself holds
  focus.

The shared KeyDown/KeyUp test helpers in AccessKeyHandlerTests have been updated to
populate Source the same way production code does
(KeyboardDevice.Instance?.FocusedElement ?? target), since they previously never set it.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None. IsFocusWithinOwner is a private method; its signature change is not observable
from outside AccessKeyHandler.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #21806

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0068103-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[AutumnYuanc]

@cla-avalonia agree

[AutumnYuanc]

Pushed a75ad36..9c0ffae addressing the review feedback:

• IsFocusWithinOwner's owner parameter is now typed as InputElement
  (matching _owner), removing the redundant is Visual check
• Fixed indentation
• Added Should_Raise_AccessKey_When_Focus_Is_On_Descendant to cover the
  IsVisualAncestorOf branch (raises KeyDown/KeyUp on a descendant
  control instead of the owner)
• Reverted the unused Source assignment in the KeyDown/KeyUp test
  helpers (it was being overwritten by EventRoute.RaiseEvent anyway)
• Shortened the comment in Should_Raise_Key_Events_For_Registered_Access_Key

@MrJul PTAL

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0068175-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0068177-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]