Reland "fix(widgets): dismiss open RawTooltip on Escape key (WCAG 1.4.13)" - #193192

Relands #192966 (reverted in #193183) with OverlayEntry disposal in raw_tooltip_test.dart and FocusManager early key-event interception in raw_tooltip.dart.

What changed since #192966 (f8c85519a060)

1. Test OverlayEntry Leak Fix (raw_tooltip_test.dart):
   • In raw_tooltip_test.dart (Escape key dismisses hovered RawTooltip even when primaryFocus is on a sibling consuming Escape), #192966 passed an inline OverlayEntry(...) to Overlay(initialEntries: <OverlayEntry>[...]) without calling entry.remove() and entry.dispose() in addTearDown. Because Overlay does not dispose initialEntries on unmount, the OverlayEntry's internal ValueNotifier<_OverlayEntryWidgetState?> was flagged as notDisposed by leak_tracker in CI (Linux framework_tests_widgets_leak_tracking).
   • This reland captures OverlayEntry? entry and registers addTearDown(() { entry?.remove(); entry?.dispose(); }). Verified locally with flutter test --dart-define=LEAK_TRACKING=true test/widgets/raw_tooltip_test.dart.
2. Stop Escape Propagation Before Focus-Tree Shortcuts (raw_tooltip.dart):
   • In #192966, RawTooltipState registered HardwareKeyboard.instance.addHandler(_handleKeyEvent) alone. Because KeyEventManager runs HardwareKeyboard handlers before FocusManager.handleKeyMessage without stopping the focus-tree walk when returning true, emptying RawTooltip._openedTooltips in HardwareKeyboard caused Escape to still reach focused siblings (siblingReceivedEscape) or WidgetsApp's Shortcuts (DismissIntent on enclosing ModalRoutes such as DatePickerDialog).
   • This reland registers a single static pair (FocusManager.instance.addEarlyKeyEventHandler(_handleEarlyKeyEvent) plus HardwareKeyboard.instance.addHandler(_handleHardwareKeyEvent) fallback when primaryFocus == null) when RawTooltip._openedTooltips transitions from empty to non-empty, and removes them when the last open tooltip closes or disposes. Returning KeyEventResult.handled from _handleEarlyKeyEvent stops FocusManager before walking the focus tree, and a second test (Escape key dismisses open RawTooltip inside WidgetsApp without invoking ancestor DismissIntent) verifies both cases.

Original description

Fixes http://b/463905826

WCAG 1.4.13 (Content on Hover or Focus) requires a mechanism to dismiss additional content triggered by pointer hover or keyboard focus (Escape key) without moving pointer hover or keyboard focus, and without unintended side effects such as dismissing an enclosing modal dialog (DatePickerDialog) when a tooltip is open.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.