HotKeyGesture: treat modifier virtual keys explicitly, remove Windows modifier, and add display tests

[stuartd]

Motivation

• Improve validation of hotkey gestures by treating physical modifier virtual-key codes explicitly and disallowing modifier keys as primary keys.

Description

• Removed the Windows flag from the HotKeyModifiers enum and replaced numeric VK literals with named constants like VirtualKeyShift, VirtualKeyControl, and VirtualKeyMenu plus left/right variants.
• Centralized modifier virtual-key detection into a ModifierVirtualKeys set and updated IsValid to reject modifier VKs and reserved Windows keys (VirtualKeyLeftWindows/VirtualKeyRightWindows) as primary keys.
• Simplified DisplayText by relying on IsValid for unassigned detection.
• Updated tests to account for the removed enum member by using a raw value for the Windows modifier and added InvalidShortcutsDisplayAsUnassigned to assert invalid gestures render as Unassigned.

Testing

• Ran the HotKeyGestureTests unit tests under tests/ClipDiff.Windows.Tests, which include DefaultShortcutIsCtrlAltD, FormatsTopRowNumpadAndFunctionKeys, RejectsUnsafeOrIncompleteShortcuts, InvalidShortcutsDisplayAsUnassigned, and InvalidPersistedShortcutFallsBackToDefault, and all tests passed.

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Codex Task