test: fix AsyncDisposeMany thread safety test timeout on CI (#1082)

Increased timeout from 5s to 30s. The original 5s was too tight
for CI runners, causing consistent timeouts on slower machines.
100K items retained for thorough coverage.

Author: dwcullop

src/DynamicData.Tests/Cache/AsyncDisposeManyFixture.IntegrationTests.cs

@@ -139,7 +139,7 @@ public async Task ItemDisposalsComplete_DisposalsCompletedOccursAndCompletes(Ite
             disposalsCompletedResults.HasCompleted.Should().BeTrue("the source and all disposals have completed");
         }
 
-        [Fact(Timeout = 5_000)]
+        [Fact(Timeout = 30_000)]
         public async Task ItemDisposalsOccurOnMultipleThreads_DisposalIsThreadSafe()
         {
             using var source = new SourceCache<AsyncDisposableItem, int>(static item => item.Id);