Fix TokenAcquirerFactory null reference exception when AppContext.BaseDirectory is root path (#3443)

* Initial plan

* Fix TokenAcquirerFactory DefineConfiguration null reference issue

Co-authored-by: pmaytak <[email protected]>

* Add comprehensive tests for DefineConfiguration null handling

Co-authored-by: pmaytak <[email protected]>

* Remove not useful tests.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: pmaytak <[email protected]>

Author: Copilot

src/Microsoft.Identity.Web.TokenAcquisition/TokenAcquirerFactory.cs

@@ -247,7 +247,8 @@ private IConfiguration ReadConfiguration()
         /// <returns>Returns the base path for configuration files</returns>
         protected virtual string DefineConfiguration(IConfigurationBuilder builder)
         {
-            return Path.GetDirectoryName(AppContext.BaseDirectory)!;
+            string? basePath = Path.GetDirectoryName(AppContext.BaseDirectory);
+            return !string.IsNullOrEmpty(basePath) ? basePath : AppContext.BaseDirectory;
         }
 
         ITokenAcquirerFactory implementation;