Fix clipped progress ring in tab when tab title is too long (#14167)

It turns out that the negative margin for the progress ring is causing
the clipping in case the tab title gets too long:

https://github.com/microsoft/terminal/blob/43dbbd590fa4b46c37e9970415f8150d3c399598/src/cascadia/TerminalApp/TabHeaderControl.xaml#L18-L27

The negative margin was introduced in #8113 because the progress ring is
supposed to replace the tab icon but the `TabView` still reserves space
even if no icon is set (see
#8133 (comment)).
However, it is not actually the `TabView` reserving space even when
there is no icon, but a workaround for a crash in the
`IconPathConverter` that returns a `BitmapIconSource` with a `nullptr`
source instead of a `nullptr` `IconSource`:

https://github.com/microsoft/terminal/blob/43dbbd590fa4b46c37e9970415f8150d3c399598/src/cascadia/TerminalSettingsModel/IconPathConverter.cpp#L143-L154

The workaround in `IconPathConverter` could probably be removed as I did
not find any instance where it is still used in a way that could trigger
the mentioned crash, but I did not dare to just remove it as I do not
know enough about the code by far. Hence, I opted to just locally
instantiate the `IconSource` with a `nullptr` directly in `TerminalTab`.

Fixes #8910

src/cascadia/TerminalApp/TabHeaderControl.xaml

@@ -20,16 +20,11 @@
                           Height="15"
                           MinWidth="0"
                           MinHeight="0"
-                          Margin="-7.5,0,8,0"
+                          Margin="3,0,8,0"
                           IsActive="{x:Bind TabStatus.IsProgressRingActive, Mode=OneWay}"
                           IsIndeterminate="{x:Bind TabStatus.IsProgressRingIndeterminate, Mode=OneWay}"
                           Visibility="{x:Bind TabStatus.IsProgressRingActive, Mode=OneWay}"
                           Value="{x:Bind TabStatus.ProgressValue, Mode=OneWay}" />
-        <!--
-            We want the progress ring to 'replace' the tab icon, but we don't have control
-            over the tab icon here (the tab view item does) - so we hide the tab icon there
-            and use a negative margin for the progress ring here to put it where the icon would be
-        -->
         <FontIcon x:Name="HeaderBellIndicator"
                   Margin="0,0,8,0"
                   FontFamily="{ThemeResource SymbolThemeFontFamily}"

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -15,6 +15,7 @@ using namespace winrt::Windows::UI::Xaml;
 using namespace winrt::Windows::UI::Core;
 using namespace winrt::Microsoft::Terminal::Control;
 using namespace winrt::Microsoft::Terminal::Settings::Model;
+using namespace winrt::Microsoft::UI::Xaml::Controls;
 using namespace winrt::Windows::System;
 
 namespace winrt
@@ -316,7 +317,7 @@ namespace winrt::TerminalApp::implementation
                 if (hide)
                 {
                     Icon({});
-                    TabViewItem().IconSource(IconPathConverter::IconSourceMUX({}));
+                    TabViewItem().IconSource(IconSource{ nullptr });
                 }
                 else
                 {