aliencube#235 Add inheritance relationship test for UpstageConnector

- Add test to verify UpstageConnector inherits from LanguageModelConnector
- Add using statement for OpenChat.PlaygroundApp.Abstractions namespace
- Implement Theory test with InlineData for both inheritance directions:
  - LanguageModelConnector.IsAssignableFrom(UpstageConnector) should be true
  - UpstageConnector.IsAssignableFrom(LanguageModelConnector) should be false
- Ensures proper polymorphism and type safety in connector architecture
- Validates adherence to Liskov Substitution Principle

Author: gremh97

test/OpenChat.PlaygroundApp.Tests/Connectors/UpstageConnectorTests.cs

@@ -1,3 +1,4 @@
+using OpenChat.PlaygroundApp.Abstractions;
 using OpenChat.PlaygroundApp.Configurations;
 using OpenChat.PlaygroundApp.Connectors;
 
@@ -25,6 +26,19 @@ private static AppSettings BuildAppSettings(string? baseUrl = BaseUrl, string? a
         };
     }
 
+    [Trait("Category", "UnitTest")]
+    [Theory]
+    [InlineData(typeof(LanguageModelConnector), typeof(UpstageConnector), true)]
+    [InlineData(typeof(UpstageConnector), typeof(LanguageModelConnector), false)]
+    public void Given_BaseType_Then_It_Should_Be_AssignableFrom_DerivedType(Type baseType, Type derivedType, bool expected)
+    {
+        // Act
+        var result = baseType.IsAssignableFrom(derivedType);
+
+        // Assert
+        result.ShouldBe(expected);
+    }
+
     [Trait("Category", "UnitTest")]
     [Fact]
     public void Given_Settings_Is_Null_When_EnsureLanguageModelSettingsValid_Invoked_Then_It_Should_Throw()