Move shared fixtures and parity utilities to TestUtilities project

- git mv JetStreamApiFixture, JetStreamClusterFixture, LeafFixture,
  Parity utilities, and TestData from NATS.Server.Tests to
  NATS.Server.TestUtilities
- Update namespaces to NATS.Server.TestUtilities (and .Parity sub-ns)
- Make fixture classes public for cross-project access
- Add PollHelper to replace Task.Delay polling with SemaphoreSlim waits
- Refactor all fixture polling loops to use PollHelper
- Add 'using NATS.Server.TestUtilities;' to ~75 consuming test files
- Rename local fixture duplicates (MetaGroupTestFixture,
  LeafProtocolTestFixture) to avoid shadowing shared fixtures
- Remove TestData entry from NATS.Server.Tests.csproj (moved to
  TestUtilities)

tests/NATS.Server.Tests/JetStream/Api/ApiEndpointParityTests.cs

@@ -1,3 +1,5 @@
+using NATS.Server.TestUtilities;
+
 // Go reference: golang/nats-server/server/jetstream.go — $JS.API.* subject dispatch
 // Covers create/info/update/delete for streams, create/info/list/delete for consumers,
 // direct-get access, account info, and 404 routing for unknown subjects.

tests/NATS.Server.Tests/JetStream/Api/ConsumerPauseApiTests.cs

@@ -3,6 +3,7 @@
 // time-bounded pauses and response body containing pause state.
 
 using NATS.Server.JetStream.Api;
+using NATS.Server.TestUtilities;
 
 namespace NATS.Server.Tests.JetStream.Api;