CmdPal: Prevent PgUp/PgDown from selecting non-internactive items (#46439)

## Summary of the Pull Request

This PR prevents paging (<kbd>PgUp</kbd>/<kbd>PgDown</kbd>) in the item
list from selecting non-interactive items (such as separators or section
headers).

It adds `FindSelectableIndex` and `FindSelectableIndexForPageNavigation`
helper methods, which locate the next interactive item in the given
direction. These methods are used to guard paging navigation and to
consolidate related logic elsewhere.

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## PR Checklist

- [x] Closes: #46283 
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **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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