remove unneeded keyProvider requirement test

Co-authored-by: lokitoth <6936551+lokitoth@users.noreply.github.com>

dotnet/tests/Microsoft.Agents.AI.Hosting.UnitTests/IsolationKeyScopedAgentSessionStoreTests.cs

@@ -53,17 +53,6 @@ public class IsolationKeyScopedAgentSessionStoreTests
             new IsolationKeyScopedAgentSessionStore(null!, provider));
     }
 
-    /// <summary>
-    /// Verify that constructor throws ArgumentNullException when keyProvider is null.
-    /// </summary>
-    [Fact]
-    public void RequiresKeyProvider()
-    {
-        // Act & Assert
-        Assert.Throws<ArgumentNullException>("keyProvider", () =>
-            new IsolationKeyScopedAgentSessionStore(this._innerStoreMock.Object, null!));
-    }
-
     /// <summary>
     /// Verify that constructor uses default options when options is null.
     /// </summary>