CmdPal: Harden ListViewModel fetch synchronization

[jiripolasek]

Summary of the Pull Request

This PR improves fetching of list items in ListViewModel:

• Fixes _vmCache concurrency with copy-on-write cache publication.
• Preserves latest-fetch-wins behavior across overlapping RPC GetItems() calls.
• Prevents stale or canceled fetches from publishing and makes them abort promptly.
• Improves cancellation cleanup for abandoned item view models and replaced token sources.
• Updates empty-state tracking to follow overlapping fetch activity correctly.
• Reduces hot-path cache overhead by removing per-item cache locking and full cache rebuilds.
• Adds guard against re-entry, to prevent situations like CmdPal: StackOverflow in Time&Date #46329:
  • Defers ItemsChanged-triggered fetches raised during GetItems() until the call unwinds;
  • Uses a thread-local reentry guard so unrelated cross-thread fetches are not delayed;
  • Adds a regression test covering recursive GetItems() refresh behavior.
  • Make sure we never invoke FetchItems on UI thread, and be loud in debug when we are.

PR Checklist

• Closes: CmdPal: Corrupted view model cache on a list #46331

• 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 for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR hardens CmdPal ListViewModel item fetching to better handle overlapping refreshes, cancellation, and re-entrant ItemsChanged scenarios (notably the recursive GetItems() stack overflow pattern in #46329/#46429).

Changes:

• Reworks fetch synchronization: generation-based “latest fetch wins”, copy-on-write VM cache publication, and more robust cancellation/cleanup.
• Adds a thread-local re-entrancy guard to defer ItemsChanged-triggered fetches raised during GetItems() until unwinding.
• Adds a regression unit test covering recursive refresh behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListViewModel.cs	Implements new fetch synchronization model (locks, generation tracking, deferred fetch), VM cache copy-on-write, and CTS cleanup helper.
src/modules/cmdpal/Tests/Microsoft.CmdPal.UI.ViewModels.UnitTests/ListViewModelTests.cs	Adds a regression test ensuring ItemsChanged during GetItems() triggers a deferred (non-recursive) subsequent fetch.

[michaeljolley]

In testing, I continually get missing icons in the All Apps list.

[jiripolasek]

In testing, I continually get missing icons in the All Apps list

I'll investigate I'm kidding, a squad of highly trained squirrels was dispatched to do that...