feat(batch5): implement jetstream error helpers and group01 constructors

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

@@ -1,6 +1,7 @@
 // Copyright 2020-2026 The NATS Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 
+using System.Reflection;
 using Shouldly;
 
 namespace ZB.MOM.NatsNet.Server.Tests.JetStream;
@@ -97,4 +98,39 @@ public sealed class JetStreamErrorsTests
             JsApiErrors.NewJSPeerRemapError(JsApiErrors.Unless(new Exception("other error"))),
             peerRemap).ShouldBeTrue();
     }
+
+    [Fact]
+    public void ParseOpts_WithUnlessApiErrorOption_ReturnsOverride()
+    {
+        var parseOpts = typeof(JsApiErrors).GetMethod(
+            "ParseOpts",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        parseOpts.ShouldNotBeNull();
+
+        var expected = new JsApiError { Code = 401, ErrCode = 2048, Description = "override" };
+        var result = parseOpts.Invoke(
+            null,
+            [new JsApiErrors.ErrorOption[] { JsApiErrors.Unless(expected) }]);
+
+        result.ShouldBeAssignableTo<JsApiError>()
+            .ErrCode.ShouldBe(expected.ErrCode);
+    }
+
+    [Fact]
+    public void ToReplacerArgs_WithStringErrorAndObject_ConvertsToStringValues()
+    {
+        var toReplacerArgs = typeof(JsApiErrors).GetMethod(
+            "ToReplacerArgs",
+            BindingFlags.NonPublic | BindingFlags.Static);
+
+        toReplacerArgs.ShouldNotBeNull();
+
+        var result = toReplacerArgs.Invoke(
+            null,
+            [new object?[] { "{string}", "value", "{error}", new InvalidOperationException("boom"), "{number}", 42 }]);
+
+        result.ShouldBeAssignableTo<string[]>()
+            .ShouldBe(["{string}", "value", "{error}", "boom", "{number}", "42"]);
+    }
 }