test(commons): failing tests for Layer B serializer + validator extensions

Adds 11 new tests covering:
- Roundtrip of DiscardOldest/SubscriptionPriority/SubscriptionDisplayName/TimestampsToReturn
- Roundtrip of UserIdentity sub-object across all three TokenTypes
- Roundtrip of Deadband sub-object
- ToFlatDict/FromFlatDict for UserIdentity.* and Deadband.* dotted keys
- Validator rules: empty SubscriptionDisplayName, UsernamePassword w/o Username,
  X509 w/o CertificatePath, Deadband Value <= 0, prefix propagation

Build passes; tests fail because serializer/validator have not been extended yet
(TDD red phase). Task B2 will implement the changes to drive them green.

tests/ScadaLink.Commons.Tests/Validators/OpcUaEndpointConfigValidatorTests.cs

@@ -109,4 +109,96 @@ public class OpcUaEndpointConfigValidatorTests
         Assert.Contains(r.Errors, e => e.EntityName == "Primary.EndpointUrl");
         Assert.Contains(r.Errors, e => e.EntityName == "Primary.QueueSize");
     }
+
+    // ── Layer B extensions: auth, deadband, subscription display name ──
+
+    [Fact]
+    public void Validate_EmptySubscriptionDisplayName_Fails()
+    {
+        var c = Valid();
+        c.SubscriptionDisplayName = "";
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.Contains(r.Errors, e => e.EntityName == "SubscriptionDisplayName");
+    }
+
+    [Fact]
+    public void Validate_UserIdentityAnonymous_NoExtraFieldsRequired()
+    {
+        var c = Valid();
+        c.UserIdentity = new OpcUaUserIdentityConfig { TokenType = OpcUaUserTokenType.Anonymous };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.True(r.IsValid);
+    }
+
+    [Fact]
+    public void Validate_UsernamePasswordWithoutUsername_Fails()
+    {
+        var c = Valid();
+        c.UserIdentity = new OpcUaUserIdentityConfig
+        {
+            TokenType = OpcUaUserTokenType.UsernamePassword,
+            Username = "",
+            Password = "secret"
+        };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.Contains(r.Errors, e => e.EntityName == "UserIdentity.Username");
+    }
+
+    [Fact]
+    public void Validate_X509WithoutCertificatePath_Fails()
+    {
+        var c = Valid();
+        c.UserIdentity = new OpcUaUserIdentityConfig
+        {
+            TokenType = OpcUaUserTokenType.X509Certificate,
+            CertificatePath = ""
+        };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.Contains(r.Errors, e => e.EntityName == "UserIdentity.CertificatePath");
+    }
+
+    [Fact]
+    public void Validate_UsernamePasswordWithUsername_Passes()
+    {
+        var c = Valid();
+        c.UserIdentity = new OpcUaUserIdentityConfig
+        {
+            TokenType = OpcUaUserTokenType.UsernamePassword,
+            Username = "alice",
+            Password = ""
+        };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.True(r.IsValid);
+    }
+
+    [Fact]
+    public void Validate_DeadbandWithNonPositiveValue_Fails()
+    {
+        var c = Valid();
+        c.Deadband = new OpcUaDeadbandConfig { Type = OpcUaDeadbandType.Absolute, Value = 0 };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.Contains(r.Errors, e => e.EntityName == "Deadband.Value");
+    }
+
+    [Fact]
+    public void Validate_DeadbandWithPositiveValue_Passes()
+    {
+        var c = Valid();
+        c.Deadband = new OpcUaDeadbandConfig { Type = OpcUaDeadbandType.Percent, Value = 1.5 };
+        var r = OpcUaEndpointConfigValidator.Validate(c);
+        Assert.True(r.IsValid);
+    }
+
+    [Fact]
+    public void Validate_UserIdentityErrorsPrefixed_WithFieldPrefix()
+    {
+        var c = Valid();
+        c.UserIdentity = new OpcUaUserIdentityConfig
+        {
+            TokenType = OpcUaUserTokenType.UsernamePassword,
+            Username = ""
+        };
+        var r = OpcUaEndpointConfigValidator.Validate(c, fieldPrefix: "Primary.");
+        Assert.Contains(r.Errors, e => e.EntityName == "Primary.UserIdentity.Username");
+    }
 }