Fix NullReferenceException in BorderText_SizeChanged for message templates

[Copilot]

The app was crashing with a NullReferenceException when requesting messages from the bot on both Android and Windows platforms. The error occurred in UserMessageItemTemplate.BorderText_SizeChanged at line 27 when trying to access _parentGrid.Width.

Root Cause:
The _parentGrid field is only initialized in the ParentGrid_SizeChanged event handler, but BorderText_SizeChanged can be triggered before this event fires, leaving _parentGrid null when accessed.

Solution:
Added defensive null checks in both UserMessageItemTemplate and BotMessageItemTemplate before accessing _parentGrid.Width:

private void BorderText_SizeChanged(object sender, EventArgs e)
{
    var borderText = (Border)sender;

    // Prevent NullReferenceException if _parentGrid hasn't been initialized yet
    if (_parentGrid == null)
        return;

    var widthMaxBorderText = _parentGrid.Width - 45;
    // ... rest of method unchanged
}

Testing:

• Created validation tests confirming the fix prevents the NullReferenceException
• Verified the method continues to work correctly when _parentGrid is properly initialized
• Changes are minimal and surgical (4 lines added per template)

This fix ensures the app no longer crashes when users send messages to the bot while maintaining all existing functionality.

Fixes #6.

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