Wait for quiescence in MergeManyChangeSets stress tests

[dwcullop]

Problem

MergeManyChangeSetsCacheSourceCompareFixture.MultiThreadedStressTest(10, 50) fails intermittently in CI with prices present in market.PricesCache.Items but missing from the live priceResults aggregator. The mechanism is the post-#1079 cache delivery model:

• Old (Synchronize(lock) shape): AddOrUpdate did not return until every subscriber had finished processing the notification. Cache state and observed notifications were always in lockstep.
• New (SharedDeliveryQueue shape): AddOrUpdate enqueues a notification and returns; the actual delivery runs later on whichever thread wins the drain. This is intentional - it removes the cross-cache deadlock the old shape produced - but it opens a small window between mutation and observed delivery.

A test that compares a live aggregator's view against cache.Items at assert time can see disagreement during that window. The stress fixture's add/remove streams are still in flight at the moment of _marketCache.Dispose; any per-market AddOrUpdate that lands after MergeManyChangeSets unsubscribes from that market's LatestPrices stream mutates PricesCache (so the post-hoc .Items snapshot sees it) without delivering a notification to priceResults (so the live aggregator never sees it).

The source-compare fixture had already adopted the right shape to defend against this:

var merged = source.MergeManyChangeSets(...).Publish();
var cacheCompleted = merged.LastOrDefaultAsync().ToTask();
using var local = merged.AsAggregator();
using var connect = merged.Connect();
// ... stress ...
await cacheCompleted;
CheckResultContents(..., local);

The two sibling stress fixtures (MergeManyChangeSetsCacheFixture and MergeManyChangeSetsListFixture) did not. They subscribed two independent chains (one for the aggregator, one for the sub/unsub loop) and asserted without waiting for either to complete.

Fix

Port the source-compare pattern to both sibling fixtures:

• Publish the merged stream so every consumer in the test shares one upstream chain.
• Capture merged.LastOrDefaultAsync().ToTask() before connecting.
• Place the verification aggregator on the Publish chain so it shares the completion semantics.
• await the completion task between the stress loop and CheckResultContents.

No production code is changed. No operator semantics change.

Verification

20 consecutive runs of MergeManyChangeSetsCacheSourceCompareFixture.MultiThreadedStressTest at xUnit.MaxParallelThreads=16, zero failures. The combined run of all three stress fixtures passes 163/163 tests per iteration.

Note on SynchronizedRandomizer

The previous commit on this branch added SynchronizedRandomizer to wrap Bogus.Randomizer with explicit synchronization. That was based on a misreading: Bogus.Randomizer takes a process-wide lock on Locker.Value for every generator call (verified via IL inspection of Number(int, int)), regardless of whether localSeed is set, so the wrapper was solving a non-problem. That commit is reverted in this push.