CmdPal: Prevent unsynchornized access to More commands (#46020)

## Summary of the Pull Request

This PR fixes a crash caused by unsynchronized access to a context menu:

- Fixes `System.ArgumentException: Destination array was not long
enough` crash in `CommandItemViewModel.AllCommands` caused by
`List<T>.AddRange()` racing with background `BuildAndInitMoreCommands`
mutations
- Replaces mutable `List<T>` public surfaces with immutable array
snapshots protected by a `Lock`; writers hold the lock, mutate the
backing list, then atomically publish new snapshots via `volatile`
fields that readers access lock-free
- Applies the same snapshot pattern to `ContentPageViewModel`, using a
bundled `CommandSnapshot` object for atomic publication (since
`PrimaryCommand` drives command invocation there, not just UI hints)
- Narrows `IContextMenuContext.MoreCommands` and `AllCommands` from
`List<T>`/`IEnumerable<T>` to `IReadOnlyList<T>` to prevent consumers
from casting back and mutating
- Moves `SafeCleanup()` calls outside locks in cleanup paths to avoid
holding the lock during cross-process RPC calls

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## PR Checklist

- [x] Closes: #45975 
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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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