Fix Korean IME arrow key inserting character at wrong position (#20039)

Fix a regression introduced in v1.24 where pressing an arrow key during
Korean IME composition caused the committed character to be inserted at
the wrong cursor position.

Before #19738, the Korean IME activated through IMM32 (excluded from TSF
by `TF_TMAE_UIELEMENTENABLEDONLY`) and was not affected by
`TermControl::_KeyHandler` forwarding keys to the PTY during
composition. After #19738, the Korean IME activates through TSF, making
a missing composition guard in `_KeyHandler` visible as a bug.

The sequence of events that causes the bug:

1. User presses Left arrow during active Korean IME composition (e.g.
composing `가`).
2. `_KeyHandler` calls `_TrySendKeyEvent(VK_LEFT)` which enqueues
`\x1b[D` to the PTY input queue. The cursor moves.
3. TSF processes the key. The Korean IME sees the arrow and ends the
composition.
4. `OnEndComposition` schedules `_doCompositionUpdate` with
`TF_ES_ASYNC`.
5. The async session fires, reads finalized text `가`, calls
`HandleOutput("가")`.
6. PTY processes `[\x1b[D, "가"]`: cursor moves left first, then `가` is
inserted at the wrong (already-moved) position.

The fix adds a guard before `_TrySendKeyEvent`, which mirrors the
existing behavior in conhost (windowio.cpp). When TSF has an active
composition, key events are not converted into input. The Korean IME
re-injects navigation and confirmation keys after the composition ends,
at which point `HasActiveComposition()` returns false and they are
forwarded normally.

**Historical note:** This guard was not needed before PR #17067 (v1.22)
because the old implementation used WinRT `CoreTextServices` via XAML
(`TSFInputControl.xaml`). The XAML framework intercepted composition key
events before `_KeyHandler`. The new custom Win32 TSF context in #17067
no longer does this. The bug was latent from v1.22 but only became
visible for Korean in v1.24 when #19738 removed
`TF_TMAE_UIELEMENTENABLEDONLY`.

## Validation Steps Performed

1. Open Windows Terminal with Korean IME (Dubeolsik layout).
2. Type `rk` to begin composing `가` (composition active, syllable not
yet committed).
3. Press the Left arrow key.
4. Before fix: `가` is inserted one cell to the left of the intended
position.
5. After fix: `가` is inserted at the correct position, then cursor moves
left.

Also verified:
- Normal Korean text input (typing without arrow keys) still works
correctly.
- Arrow key navigation when no composition is active still works
correctly.
- English and other IME input is not affected.

Closes #20038
Refs #19738

src/cascadia/TerminalControl/TermControl.cpp

@@ -1795,6 +1795,16 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             return true;
         }
 
+        // While TSF has an active composition, key events must not be forwarded
+        // to the PTY directly. The IME re-injects navigation/confirmation keys
+        // after composition ends, at which point HasActiveComposition() == false
+        // and they are forwarded normally. This mirrors conhost v1 behavior in
+        // src/interactivity/win32/windowio.cpp.
+        if (GetTSFHandle().HasActiveComposition())
+        {
+            return true;
+        }
+
         if (_TrySendKeyEvent(vkey, scanCode, modifiers, keyDown))
         {
             return true;