fix: update test override signature for CreateBrowserCredential loginHint parameter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sellakumaran

src/Tests/Microsoft.Agents.A365.DevTools.Cli.Tests/Services/AuthenticationServiceTests.cs

@@ -800,7 +800,7 @@ public TestableAuthenticationService(
             _deviceCodeCredential = deviceCodeCredential;
         }
 
-        protected override TokenCredential CreateBrowserCredential(string clientId, string tenantId)
+        protected override TokenCredential CreateBrowserCredential(string clientId, string tenantId, string? loginHint = null)
             => _browserCredential;
 
         protected override TokenCredential CreateDeviceCodeCredential(string clientId, string tenantId)