fix(textinput): show touch keyboard on focus and on tap (windowless island; re-tap reopens)

[FaithfulAudio]

Fixes #16331.

Problem

On a Windows tablet with no physical keyboard, tapping a single-line <TextInput> (e.g. a login field) never raises the touch keyboard. Additionally, after the keyboard is dismissed, re-tapping the (still-focused) field does not bring it back.

Root cause

Fabric/Composition TextInput (WindowsTextInputComponentView) has no code that requests the touch keyboard on focus — onGotFocus never touches InputPane/CoreInputView. In a WinAppSDK Win32 island app there is no CoreWindow, and the RichEdit is hosted windowless via ITextServices (no child edit HWND), so the OS's own touch-keyboard auto-invoke heuristic never fires either. Net: the keyboard never appears. (The UWP-era PR #2029 for this was closed and relied on GetForCurrentView() APIs that don't apply to the island model.)

A tap on an already-focused field does not fire onGotFocus, so a focus-only trigger can't re-raise a keyboard the user has dismissed.

Fix

New helper ShowSoftKeyboard() requests the touch keyboard via InputPane, obtained through the Win32 interop IInputPaneInterop::GetForWindow() keyed off the island's top-level AppWindow HWND (rootComponentView()->GetHwndForParenting()). Gated on slate/tablet posture (SM_CONVERTIBLESLATEMODE == 0) so it does not pop on a laptop/convertible with a keyboard attached; wrapped in try/catch so it can never break input handling.

It is called from both:

• onGotFocus — raise the keyboard when the field gains focus, and
• OnPointerPressed — raise it on tap, so re-tapping an already-focused field reopens a dismissed keyboard.

Focus is intentionally left untouched on keyboard dismissal (the field stays focused; the user reopens the keyboard by tapping it again — the platform-appropriate behavior here).

Uses IInputPaneInterop from <inputpaneinterop.h> and winrt::Windows::UI::ViewManagement::InputPane — both ship in the Windows SDK already referenced by Microsoft.ReactNative, so there are no .vcxproj/reference changes.

Validation

Compiled into Microsoft.ReactNative from source (yarn-patch framework build, UseExperimentalNuget=false) in a production RNW 0.83.2 new-arch app (Facilitron FIT), x64 Release, deployed to a physical x64 tablet. With the keyboard detached (Windows suppresses the touch keyboard while a hardware keyboard is attached):

• tapping the login field raises the touch keyboard ✅
• dismissing the keyboard keeps the field focused ✅
• re-tapping the field reopens the keyboard ✅

All three confirmed on-device. Remaining for upstream: CI build across archs.

Caveats for reviewers

• Authored against 0.83-stable to match our production app; happy to re-cut onto main.
• The posture gate uses SM_CONVERTIBLESLATEMODE (non-convertible desktops report slate/0). Open to a different signal (e.g. UserInteractionMode/touch-initiated focus) if preferred.
• Mirrors the existing TextInput fixes (fix(textinput): center single-line text correctly at >100% display scale #16302, fix(textinput): correct placeholder layout constraints (px vs DIP) and no-op NaN fontSize guard #16303).

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