Refine hotkey validation: disallow Windows modifier and modifier virtual keys, add VK constants and tests

[stuartd]

Motivation

• Prevent assigning modifier keys or the Windows modifier bit as a primary shortcut key and disallow unsafe/incomplete shortcuts like plain Shift or Alt+F4.
• Make virtual-key handling explicit and robust by naming VK constants and centralizing modifier virtual-key detection.
• Improve validation rules to enforce sane VK boundaries and supported modifier combinations.

Description

• Remove the Windows flag from HotKeyModifiers and stop treating the Windows modifier as supported.
• Add named VirtualKey* constants and build a ModifierVirtualKeys set to explicitly recognize keys that are modifiers and therefore cannot be the primary key.
• Tighten IsValid to require at least one primary modifier (Control or Alt), reject unsupported modifier bits, enforce VK range bounds, exclude Windows key VKs and modifier VKs, and disallow Alt+F4.
• Remove a redundant DisplayText special-case and rely on IsValid, and expand/adjust unit tests in HotKeyGestureTests to cover boundary cases and unsupported flags.

Testing

• Ran the updated unit test file tests/ClipDiff.Windows.Tests/HotKeyGestureTests.cs via dotnet test for the ClipDiff.Windows.Tests project.
• All tests in HotKeyGestureTests passed successfully.

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Codex Task