Fix memory leaks from missing event unsubscriptions

[Copilot]

Description of Change

Audit of event subscription patterns across the codebase revealed three memory leaks where event handlers were never properly unsubscribed:

• SearchBarHandler.cs (iOS) — Copy-paste bug: OnSearchButtonClicked was unsubscribed from CancelButtonClicked instead of SearchButtonClicked, so the search button handler was never cleaned up.

- platformView.CancelButtonClicked -= OnSearchButtonClicked;
+ platformView.SearchButtonClicked -= OnSearchButtonClicked;

• CameraSession.cs (iOS) — PreviewView.OnTapToFocus was subscribed in ConfigureAndStart() but missing from StopCameraSession() cleanup (the adjacent OnZoomChanged was correctly unsubscribed).

• SlidableLayout.cs — TapGestureRecognizer was a local variable in the constructor, so its Tapped handler could never be unsubscribed in OnHandlerChanging(). Promoted to field m_tapGestureRecognizer alongside the existing m_panGestureRecognizer.

Todos

• I have tested on an Android device.
• I have tested on an iOS device.
• I have supported accessibility

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