Use weak events for SwipeView and ShellContent child subscriptions

[wbv-profile]

Description of Change

SwipeView and ShellContent both subscribe to a child's PropertyChanged in OnChildAdded and unsubscribe in OnChildRemoved, using plain CLR handlers. A child can outlive the parent that hosted it — cached by the app, or re-parented onto a different host — and the handler then keeps every parent the child was ever attached to alive, along with that parent's whole subtree.

Both now route the subscription through the existing WeakNotifyPropertyChangedProxy, tracking one proxy per child so removal still detaches deterministically, plus the finalizer the proxy contract requires.

This is the pattern the surrounding code already reaches for. SwipeView.OnChildAdded carries a comment explaining that SwipeItems children are deliberately not subscribed for exactly this reason, and OnSwipeItemsChanged documents the earlier fix for #35481 where captured closures kept shared SwipeItems from being collected. This change extends the same reasoning to ordinary content children.

Same defect class as #38397, #38403, #38405, #38408, #38410 and #38412.

Tests

src/Controls/tests/Core.UnitTests/ContentWeakEventTests.cs adds four tests: one leak test per control (a retained Label assigned as SwipeView.Content, a retained ContentPage assigned as ShellContent.Content), plus two guards pinning that attached content is still parented and that removing it still detaches.

Reverting the src/Controls/src/Core changes fails exactly the two leak tests and passes both guards. With the fix all four pass and the full Controls.Core.UnitTests suite is green (5651 passed, 0 failed).

Issues Fixed

Fixes #37217
Fixes #37218

[github-actions]

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Hey there @wbv-profile! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[kubaflo]

Closing as overlapping with #37390 (ShellContent, 26233a1) and #38101 (SwipeView, dba446a), both already merged into inflight/current.

The event-retention diagnosis is correct, but those existing changes already replace these strong child subscriptions. Their single lazy proxy per control also matches the single relevant content/page lifecycle without the dictionaries of proxies introduced here. There is no remaining product fix here that justifies a parallel implementation.

If carrying tests forward, SwipeViewStillTracksContentIsEnabled should assert delivery to an attached handler: its current Parent-only assertion cannot detect a dropped IsEnabled notification. main still lacks the existing fixes; any deliberate backport should reuse the inflight changes. Thanks for identifying these retention paths.