Stop the app-model tests racing the model's own callbacks

[FrodeHus]

What this changes

Elevate.App.Tests failed roughly once in twenty runs with:

System.InvalidOperationException : Operations that change non-concurrent collections
must have exclusive access. A concurrent update was performed on this collection and
corrupted its state.
  at System.Collections.Generic.Dictionary`2.set_Item
  at AppModel.ActivateCoreAsync ... AppModel.Activation.cs:line 172

AppModel captures SynchronizationContext.Current and marshals the activation coordinator's
progress callback back onto it, because in the app that is the one UI thread every member is
required to run on. The context xUnit installs is not a thread at all — AsyncTestSyncContext.Post
hands the callback to the thread pool — so that callback could mutate Progress and Active while
the authoritative loop at the end of ActivateCoreAsync was writing the same dictionaries.

Nothing is wrong with the app. There _context is the dispatcher, genuinely single-threaded, so
the callback and the loop are serialised; the comment on that callback already allows it to land on
either side of the loop. Only the harness broke the precondition the model documents.

The fix is one hunk in TestModel: construct the model with no ambient context, so Post runs
inline. The coordinator raises progress from inside the call the model is awaiting, so the callback
lands before that await returns — one of the two orderings the UI thread already produces, and the
deterministic one. No production code changes.

Checklist

• swift test passes in macos/ (ElevateCore) — untouched, no Swift in this diff
• dotnet test Elevate.Cli.sln passes in cli/ when the CLI or Elevate.Core changed — untouched
• xcodebuild ... test passes for ElevateAppTests — untouched
• No Elevate.xcodeproj committed
• No real client ids, tenant ids, tokens or account names in the diff
• Documentation updated where behaviour changed — no behaviour change, test-only
• CHANGELOG.md updated under ## [Unreleased] — not user-visible, so no entry

How it was verified

QuickActivateUsesTheRememberedReasonOrAsksForTheDialog is where it surfaced. Before: 2 failures in
25 runs of that filter. After: 0 in 40 consecutive runs, and the full Elevate.sln suite clean
5 times over (Elevate.Core.Tests 497, Elevate.App.Tests 165).

The race was diagnosed rather than guessed at: a scratch test confirmed the ambient context under
xUnit is Xunit.Sdk.AsyncTestSyncContext, and the stack trace names the exact dictionary write.

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