CmdPal: Harden ListViewModel fetch synchronization (#46429)

## 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 #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.

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## PR Checklist

- [x] Closes: #46331
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- [ ] **Tests:** Added/updated and all pass
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## Detailed Description of the Pull Request / Additional comments

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