[Keyboard Manager] Fix stuck modifiers and dropped key-to-text remaps

[crutkas]

Summary

Fixes stuck modifier keys and silently-dropped remaps on Keyboard Manager's single key → text path, and adds unit coverage (including a mockable injection-failure seam).

What this changes

1. Insert a dummy key event before releasing held modifiers. Releasing a lone Win or Alt key-up otherwise triggers the Start Menu / menu bar. The dummy key absorbs it so the release is inert. The dummy + releases are only injected when a modifier is actually held.
2. Accept WM_SYSKEYDOWN as well as WM_KEYDOWN. While Alt is held the system delivers WM_SYSKEYDOWN, so the previous WM_KEYDOWN-only guard silently dropped the remap whenever Alt was down.
3. Route Helpers::SendTextInput through InputInterface instead of calling Win32 SendInput directly. Besides making the path mockable, this stops the existing unit tests from injecting real keystrokes into the OS during a test run. Text is still flushed per character to preserve the existing batching workaround.
4. Never re-press released modifiers. Once a modifier key-up is injected, GetAsyncKeyState reports it as up, so re-pressing risks leaving it stuck if the user let go during injection. Leaving it released is always safe.

Testing

• New MockedInput failure seam (SetSendVirtualInputShouldFail).
• RemappedKey_ShouldPassOriginalKeyThrough_WhenInjectionFails — verifies the original key is passed through when injection fails (the core stuck-key behavior, previously untestable because the mock always succeeded).
• HandleSingleKeyToTextRemapEvent_ShouldFireAndReleaseAlt_WhenAltIsHeld — covers fix fix a couple typos #2 by asserting the remap still fires (and releases the held Alt) when the key arrives as WM_SYSKEYDOWN.
• Full Keyboard Manager engine suite: 98/98 passing, Release x64, against current main.

This is one of a small set of related "stuck key" hardening fixes; each stands alone.

[github-actions]

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