Fix TokenAcquirerFactory null reference exception when AppContext.BaseDirectory is root path

[Copilot]

This PR fixes a regression introduced in version 3.9.1 where TokenAcquirerFactory.GetDefaultInstance() throws ArgumentNullException when AppContext.BaseDirectory is a root directory.

Problem

In version 3.9.1, the DefineConfiguration method was changed from using Assembly.Location to AppContext.BaseDirectory:

Before (v3.9.0):

Assembly assembly = Assembly.GetEntryAssembly() ?? Assembly.GetExecutingAssembly();
return Path.GetDirectoryName(assembly.Location);

After (v3.9.1):

return Path.GetDirectoryName(AppContext.BaseDirectory)!;

The issue occurs because:

• Assembly.Location returns the full path to the assembly file (e.g., "C:\MyApp\bin\MyApp.dll")
• AppContext.BaseDirectory returns a directory path (e.g., "C:\" or "/")
• When AppContext.BaseDirectory is a root directory, Path.GetDirectoryName() returns null
• This null value causes SetBasePath(null) to throw ArgumentNullException

Solution

Modified the DefineConfiguration method to handle the null case by falling back to the original AppContext.BaseDirectory:

protected virtual string DefineConfiguration(IConfigurationBuilder builder)
{
    string? basePath = Path.GetDirectoryName(AppContext.BaseDirectory);
    return !string.IsNullOrEmpty(basePath) ? basePath : AppContext.BaseDirectory;
}

This ensures that:

• Normal cases continue to work as expected (e.g., "/app/bin/" → "/app/bin")
• Root directory cases are handled gracefully (e.g., "/" → "/" when Path.GetDirectoryName("/") returns null)
• The method never returns null or empty string

Testing

Added comprehensive unit tests that verify:

• The method handles null results from Path.GetDirectoryName()
• Various edge cases with root paths work correctly
• Normal directory paths continue to function as before

The fix is minimal and surgical, changing only the problematic logic while preserving all existing functionality.

Fixes #3425.

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[pmaytak]

Comment here: https://github.com/AzureAD/microsoft-identity-web/pull/3443/files#r2258453008