[UI tests] Add accessibility IDs to Command Palette UI components for improved UI testing (#41295)

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## Summary of the Pull Request
Finding elements by name is often unstable; adding an accessibility ID
is more reliable for UI tests.

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## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/SearchBar.xaml

@@ -22,6 +22,7 @@
         MinHeight="32"
         VerticalAlignment="Stretch"
         VerticalContentAlignment="Stretch"
+        AutomationProperties.AutomationId="MainSearchBox"
         KeyDown="FilterBox_KeyDown"
         PlaceholderText="{x:Bind CurrentPageViewModel.PlaceholderText, Converter={StaticResource PlaceholderTextConverter}, Mode=OneWay}"
         PreviewKeyDown="FilterBox_PreviewKeyDown"