fix(devtools): clear highlight when mouse leaves DevTools panel

[petertdinh]

Fixes #17855

When hovering a component in the DevTools Components inspector, a highlight overlay appears on the inspected page. The highlight is cleared via onMouseLeave on the tree container div. But this React synthetic event only fires when the pointer transitions between elements within the same document. When the user moves their mouse out of the DevTools panel window entirely (e.g. to the browser viewport), no element in the React tree receives mouseleave, so clearHostInstanceHighlight is never sent over the bridge and the overlay persists on the page.

The fix adds a native mouseleave listener on the DevTools panel's ownerDocument in Tree.js. When the pointer exits the panel viewport, it fires clearHighlightHostInstance and removes the overlay. Using ownerDocument (rather than document) is consistent with the existing pattern in Tree.js for browser extension compatibility.

How did you test this change?

Tested manually using the Chrome extension:

1. Opened React DevTools → Components tab on a React app
2. Hovered a component in the tree — highlight appeared on the page ✓
3. Moved the mouse out of the DevTools panel into the browser viewport — highlight cleared immediately ✓ (previously it persisted)
4. Moved the mouse back into the panel and hovered a component — highlighting still works normally ✓
5. Unhovered within the panel — highlight still clears correctly ✓

Ran the DevTools test suite: yarn test --no-watchman ReactDevTools — all tests pass.

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