Fix #916 — Memory tab tooltip stops working after returning to tab

[erikdarlingdata]

Summary

• WPF Popup (used by ChartHoverHelper) can wedge when the user navigates away from a TabItem mid-hover: TabControl unloads the parent without firing MouseLeave, so IsOpen stays true with a stale placement anchor. On return, OnMouseMove sets IsOpen = true again, but since it is already true the assignment is a no-op and the popup never re-anchors or appears.
• Memory tab is the most visible victim because it hosts 6 charts inside a nested TabControl, but the underlying bug is in the shared helper.
• Force-close the popup on chart Loaded / Unloaded / IsVisibleChanged, and toggle IsOpen off→on inside OnMouseMove so WPF re-evaluates placement even when the popup believes it is already open.

Fixes #916

Test plan

• Open Dashboard → Overview → Resource Overview → confirm tooltip appears
• Switch to Memory → Memory Overview → confirm tooltip appears
• Switch back to Overview → Resource Overview → tooltip still works
• Switch back to Memory → Memory Overview → tooltip works (regression scenario from issue)
• Repeat across all Memory sub-tabs (Memory Grants, Memory Clerks, Plan Cache, Memory Pressure Events)
• Sanity-check tooltips still behave on other tabs that use ChartHoverHelper (Resource Metrics, System Events, Queries history windows)

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