feat: add JoeAppEngine OPC UA nodes, fix DCL auto-reconnect and quality push

- Add JoeAppEngine folder to OPC UA nodes.json (BTCS, AlarmCntsBySeverity, Scheduler/ScanTime)
- Fix DataConnectionActor: capture Self in PreStart for use from non-actor threads,
  preventing Self.Tell failure in Disconnected event handler
- Implement InstanceActor.HandleConnectionQualityChanged to mark attributes Bad on disconnect
- Fix LmxFakeProxy TagMapper to serialize arrays as JSON instead of "System.Int32[]"
- Allow DataType and DataSourceReference updates in TemplateService.UpdateAttributeAsync
- Update test_infra_opcua.md with JoeAppEngine documentation

tests/ScadaLink.NotificationService.Tests/OAuth2TokenServiceTests.cs

@@ -0,0 +1,139 @@
+using System.Net;
+using System.Text.Json;
+using Microsoft.Extensions.Logging.Abstractions;
+using NSubstitute;
+
+namespace ScadaLink.NotificationService.Tests;
+
+/// <summary>
+/// Tests for OAuth2 token flow — token acquisition, caching, and credential parsing.
+/// </summary>
+public class OAuth2TokenServiceTests
+{
+    private static HttpClient CreateMockHttpClient(HttpStatusCode statusCode, string responseJson)
+    {
+        var handler = new MockHttpMessageHandler(statusCode, responseJson);
+        return new HttpClient(handler);
+    }
+
+    private static IHttpClientFactory CreateMockFactory(HttpClient client)
+    {
+        var factory = Substitute.For<IHttpClientFactory>();
+        factory.CreateClient(Arg.Any<string>()).Returns(client);
+        return factory;
+    }
+
+    [Fact]
+    public async Task GetTokenAsync_ReturnsAccessToken_FromTokenEndpoint()
+    {
+        var tokenResponse = JsonSerializer.Serialize(new
+        {
+            access_token = "mock-access-token-12345",
+            expires_in = 3600,
+            token_type = "Bearer"
+        });
+
+        var client = CreateMockHttpClient(HttpStatusCode.OK, tokenResponse);
+        var factory = CreateMockFactory(client);
+        var service = new OAuth2TokenService(factory, NullLogger<OAuth2TokenService>.Instance);
+
+        var token = await service.GetTokenAsync("tenant123:client456:secret789");
+
+        Assert.Equal("mock-access-token-12345", token);
+    }
+
+    [Fact]
+    public async Task GetTokenAsync_CachesToken_OnSubsequentCalls()
+    {
+        var tokenResponse = JsonSerializer.Serialize(new
+        {
+            access_token = "cached-token",
+            expires_in = 3600,
+            token_type = "Bearer"
+        });
+
+        var handler = new CountingHttpMessageHandler(HttpStatusCode.OK, tokenResponse);
+        var client = new HttpClient(handler);
+        var factory = CreateMockFactory(client);
+        var service = new OAuth2TokenService(factory, NullLogger<OAuth2TokenService>.Instance);
+
+        var token1 = await service.GetTokenAsync("tenant:client:secret");
+        var token2 = await service.GetTokenAsync("tenant:client:secret");
+
+        Assert.Equal("cached-token", token1);
+        Assert.Equal("cached-token", token2);
+        Assert.Equal(1, handler.CallCount); // Only one HTTP call should be made
+    }
+
+    [Fact]
+    public async Task GetTokenAsync_InvalidCredentialFormat_ThrowsInvalidOperationException()
+    {
+        var client = CreateMockHttpClient(HttpStatusCode.OK, "{}");
+        var factory = CreateMockFactory(client);
+        var service = new OAuth2TokenService(factory, NullLogger<OAuth2TokenService>.Instance);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            () => service.GetTokenAsync("invalid-no-colons"));
+    }
+
+    [Fact]
+    public async Task GetTokenAsync_HttpFailure_ThrowsHttpRequestException()
+    {
+        var client = CreateMockHttpClient(HttpStatusCode.Unauthorized, "Unauthorized");
+        var factory = CreateMockFactory(client);
+        var service = new OAuth2TokenService(factory, NullLogger<OAuth2TokenService>.Instance);
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => service.GetTokenAsync("tenant:client:secret"));
+    }
+
+    /// <summary>
+    /// Simple mock HTTP handler that returns a fixed response.
+    /// </summary>
+    private class MockHttpMessageHandler : HttpMessageHandler
+    {
+        private readonly HttpStatusCode _statusCode;
+        private readonly string _responseContent;
+
+        public MockHttpMessageHandler(HttpStatusCode statusCode, string responseContent)
+        {
+            _statusCode = statusCode;
+            _responseContent = responseContent;
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            return Task.FromResult(new HttpResponseMessage(_statusCode)
+            {
+                Content = new StringContent(_responseContent)
+            });
+        }
+    }
+
+    /// <summary>
+    /// Mock HTTP handler that counts invocations.
+    /// </summary>
+    private class CountingHttpMessageHandler : HttpMessageHandler
+    {
+        private readonly HttpStatusCode _statusCode;
+        private readonly string _responseContent;
+        public int CallCount { get; private set; }
+
+        public CountingHttpMessageHandler(HttpStatusCode statusCode, string responseContent)
+        {
+            _statusCode = statusCode;
+            _responseContent = responseContent;
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            CallCount++;
+            return Task.FromResult(new HttpResponseMessage(_statusCode)
+            {
+                Content = new StringContent(_responseContent)
+            });
+        }
+    }
+}