[PresentationFramework] Tooltip safe-area handling can throw during hover transitions

# [PresentationFramework] Tooltip safe-area handling can throw during hover transitions

## Summary
Tooltip processing can throw when pointer movement intersects with a tooltip whose underlying window/source is disposed during hover transitions.

## Observed behavior
- In tooltip-heavy UI paths, moving the mouse near or over tooltip boundaries can trigger an exception in tooltip coordinate processing.
- The failure is intermittent and timing-sensitive, so it can be difficult to reproduce deterministically.

## Repro notes
1. Run a WPF app with controls that frequently show/hide tooltips.
2. Move the mouse quickly across controls while tooltips are opening/closing.
3. Observe occasional exceptions originating from tooltip safe-area coordinate conversion paths.

## Impact
- This can crash or destabilize applications during normal tooltip interaction.
- Apps with dense UI surfaces and frequent tooltip activity are more likely to be affected.

## Expected behavior
Tooltip transitions should remain safe even if the tooltip's underlying source/window is disposed mid-flow.

## Actual behavior
The tooltip path may still use a previously captured safe-area source that has already been disposed, causing coordinate conversion to fail and throw.

## Suspected root cause
Safe-area checks only validated presence (`SafeArea != null`) and did not validate whether the underlying `PresentationSource` was still alive when used.

## Proposed fix direction
- Introduce a validity gate for safe-area usage, not just null checks.
- Treat disposed safe-area sources as invalid and fall back to behavior equivalent to "no safe area".
- Route tooltip mouse-trigger logic and safe-area exit checks through the new validity gate.
- Expose convex hull/source validity in a minimal internal property used by tooltip logic.