feat(p7-09): JetStream unit tests — versioning (12), dirstore (12), batching/errors deferred (66)

Port session P7-09: add tests from jetstream_versioning_test.go (T:1791–1808),
dirstore_test.go (T:285–296), jetstream_batching_test.go (T:716–744),
jetstream_errors_test.go (T:1381–1384), and accounts_test.go (T:80–110).

- JetStreamVersioningTests: 12 active unit tests + 6 deferred (server-required)
- DirectoryStoreTests: 12 filesystem tests using fake JWTs (no NKeys dependency)
- JetStreamBatchingTests: 29 deferred stubs (all require running JetStream cluster)
- JetStreamErrorsTests: 4 deferred stubs (NewJS* factories not yet ported)
- accounts_test.go T:80–110: 31 deferred (all use RunServerWithConfig)

Fix DirJwtStore.cs expiration bugs:
  - Use DateTimeOffset.UtcNow.UtcTicks (not Unix-relative ticks) for expiry comparison
  - Replace in-place JwtItem mutation with new-object replacement so DrainStale
    can detect stale heap entries via ReferenceEquals check

Add JetStreamVersioning.cs methods: SetStaticStreamMetadata,
SetDynamicStreamMetadata, CopyStreamMetadata, SetStaticConsumerMetadata,
SetDynamicConsumerMetadata, SetDynamicConsumerInfoMetadata, CopyConsumerMetadata.

Tests: 725 pass, 53 skipped/deferred, 0 failures.
DB: +24 complete, +66 deferred.

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/JetStream/JetStreamErrorsTests.cs

@@ -0,0 +1,50 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Mirrors server/jetstream_errors_test.go in the NATS server Go source.
+//
+// All 4 tests are deferred:
+//   T:1381 — TestIsNatsErr: uses IsNatsErr(error, ...) where the Go version accepts
+//             arbitrary error interface values (including plain errors.New("x") which
+//             evaluates to false). The .NET JsApiErrors.IsNatsError only accepts JsApiError?
+//             and the "NewJS*" factory constructors (NewJSRestoreSubscribeFailedError etc.)
+//             that populate Description templates from tags have not been ported yet.
+//   T:1382 — TestApiError_Error: uses ApiErrors[JSClusterNotActiveErr].Error() — the Go
+//             ApiErrors map and per-error .Error() method (returns "description (errCode)")
+//             differs from the .NET JsApiErrors.ClusterNotActive.ToString() convention.
+//   T:1383 — TestApiError_NewWithTags: uses NewJSRestoreSubscribeFailedError with tag
+//             substitution — factory constructors not yet ported.
+//   T:1384 — TestApiError_NewWithUnless: uses NewJSStreamRestoreError, Unless() helper,
+//             NewJSPeerRemapError — not yet ported.
+
+namespace ZB.MOM.NatsNet.Server.Tests.JetStream;
+
+/// <summary>
+/// Tests for JetStream API error types and IsNatsErr helper.
+/// Mirrors server/jetstream_errors_test.go.
+/// All tests deferred pending port of Go factory constructors and tag-substitution system.
+/// </summary>
+public sealed class JetStreamErrorsTests
+{
+    [Fact(Skip = "deferred: NewJS* factory constructors and IsNatsErr(error) not yet ported")] // T:1381
+    public void IsNatsErr_ShouldSucceed() { }
+
+    [Fact(Skip = "deferred: ApiErrors map and .Error() method not yet ported")] // T:1382
+    public void ApiError_Error_ShouldSucceed() { }
+
+    [Fact(Skip = "deferred: NewJSRestoreSubscribeFailedError with tag substitution not yet ported")] // T:1383
+    public void ApiError_NewWithTags_ShouldSucceed() { }
+
+    [Fact(Skip = "deferred: NewJSStreamRestoreError / Unless() helper not yet ported")] // T:1384
+    public void ApiError_NewWithUnless_ShouldSucceed() { }
+}