Fix web context-menu client lifecycle in SelectableRegion - #189587

On web, PlatformSelectableRegionContextMenu._activeClient — the static SelectionContainerDelegate that the right-click mousedown listener reads — can end up pointing at a detached or disposed delegate, because of three lifecycle defects:

1. detach has an inverted condition — if (_activeClient != client) { _activeClient = null; }. Detaching the active client leaves it registered, while detaching an unrelated client wipes another region's registration.
2. SelectableRegionState._handleFocusChanged calls attach unconditionally at the end of the method, so a region that just lost focus is immediately re-registered.
3. SelectableRegionState.dispose never detaches, so the static can keep pointing at a disposed delegate.

Together these produce a crash on web when the user right-clicks during a route transition: the listener calls getTransformTo(null) on the stale delegate and hits "getTransformTo cannot be called before SelectionContainer is laid out" (selection_container.dart:303). We hit this in a production web app.

This PR flips the detach condition, moves the re-attach into the focused branch, and detaches in dispose before disposing the delegate. It also adds a @visibleForTesting debugActiveClient getter, mirroring the existing debugOverrideRegisterViewFactory pattern, since the delegate is otherwise unobservable from tests. Three new browser tests in selectable_region_context_menu_test.dart isolate the defects; each is red before the fix and green after it.

A follow-up commit hardens the detach paths against BrowserContextMenu.enabled changing at runtime. Attach stays gated on the full dynamic check, but the focus-loss and dispose detach paths now gate on kIsWeb alone, so a delegate attached while the menu was enabled is still detached after the menu is disabled. This is safe because detach is identity-selective: calling it for a never-attached delegate is a no-op. Two more browser tests cover unfocus and disposal after BrowserContextMenu.disableContextMenu(), bringing it to five lifecycle tests, each red before its fix and green after.

Fixes #189575

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