[CI] Sign YamlDotNet.dll (ShortcutGuide V2 dependency) (#48050)

## Summary

Fixes the **stable** signed release pipeline failure (Dart build
[`147590319`](https://microsoft.visualstudio.com/Dart/_build/results?buildId=147590319))
at the *"Verify all binaries are signed and versioned"* task on both
`Build Release_x64` and `Build Release_arm64`:

```
Not Signed: + …\extractedMachineMsi\File\WinUI3ApplicationsFiles_File_YamlDotNet.dll
```

## Root cause

PR #40834 (*Shortcut Guide V2*) added a `YamlDotNet` `PackageReference`
to:

- `src/modules/ShortcutGuide/ShortcutGuide.Ui/ShortcutGuide.Ui.csproj`
-
`src/modules/ShortcutGuide/ShortcutGuide.IndexYmlGenerator/ShortcutGuide.IndexYmlGenerator.csproj`

Both projects publish into `WinUI3Apps\`, so `YamlDotNet.dll` is
harvested into the MSI by
`installer/PowerToysSetupVNext/generateAllFileComponents.ps1`. The PR
updated `ESRPSigning_core.json` for its own new binaries but missed this
3rd-party transitive dependency, so ESRP never signs it and
`.pipelines/versionAndSignCheck.ps1` correctly fails the build.

## Fix

One additive line to `.pipelines/ESRPSigning_core.json`, placing
`WinUI3Apps\YamlDotNet.dll` next to the other 3rd-party `WinUI3Apps\`
entries (`Google.Apis.*`, `Google.GenAI.dll`, `ReverseMarkdown.dll`,
`SharpCompress.dll`, …).

## Validation

- `git diff` shows exactly one additive line.
- File still parses as valid JSON (`ConvertFrom-Json` round-trip OK).
- Both ShortcutGuide csprojs target
`$(Platform)\$(Configuration)\WinUI3Apps`, so a single
`WinUI3Apps\YamlDotNet.dll` entry covers all uses.

## Follow-up

After merge to `main`, cherry-pick / merge into `stable` so build
`147590319` can be re-queued and the 0.100 release pipeline can proceed.

Co-authored-by: Copilot <Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.pipelines/ESRPSigning_core.json

@@ -391,6 +391,7 @@
                 "WinUI3Apps\\Google.Apis.Auth.dll",
                 "WinUI3Apps\\Google.Apis.Core.dll",
                 "WinUI3Apps\\Google.GenAI.dll",
+                "WinUI3Apps\\YamlDotNet.dll",
 
                 "boost_regex-vc143-mt-gd-x32-1_87.dll",
                 "boost_regex-vc143-mt-gd-x64-1_87.dll",