Utilize ShellName and Add Test to Enable Completions W/out Shell on Unix

src/System.CommandLine.StaticCompletions/CompletionsCommand.cs

@@ -67,8 +67,8 @@ static IShellProvider LookupShellFromEnvironment(Dictionary<string, IShellProvid
             {
                 throw new InvalidOperationException(Strings.ShellDiscovery_ShellEnvironmentNotSet);
             }
-            var shellName = Path.GetFileName(shellPath);
-            if (shellMap.TryGetValue(shellPath, out var shellProvider))
+            var shellName = Path.GetFileName(shellPath).ToLower();
+            if (shellMap.TryGetValue(shellName, out var shellProvider))
             {
                 return shellProvider;
             }

test/dotnet.Tests/CompletionTests/ShellDiscoveryTests.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable disable
+
+
+using System.CommandLine.StaticCompletions.Shells;
+using Microsoft.DotNet.Cli.Commands.BuildServer.Shutdown;
+using Xunit;
+using Xunit.Abstractions;
+using Parser = Microsoft.DotNet.Cli.Parser;
+
+namespace System.CommandLine.StaticCompletions.Tests;
+
+public class ShellDiscoveryTests()
+{
+    [Fact]
+    public void StaticCompletionsCanParseWithoutAShell()
+    {
+        var result = Parser.Instance.Parse(@"dotnet completions script");
+        result.Errors.Should().BeEmpty();
+        result.GetValue<IShellProvider>("@CompletionsCommand_ShellArgument_Description").Should().NotBeNull();
+    }
+}