dotnet completions script produces confusing error when no shell supplied

[slang25]

Describe the bug

When running dotnet completions script in the fish shell, presumably there is some default argument handling that infers that the shell is "fish", however the CLI outputs the following error:

To Reproduce

In fish shell, run just dotnet completions script, omitting the shell argument.

Exceptions (if any)

System.InvalidOperationException: The shell application 'fish' is not supported. The following shells are supported by this application: bash, pwsh, fish, zsh, nushell
   at System.CommandLine.StaticCompletions.CompletionsCommand.<.ctor>g__LookupShellFromEnvironment|2_0(Dictionary`2 shellMap)
   at System.CommandLine.StaticCompletions.CompletionsCommand.<>c__DisplayClass2_0.<.ctor>b__3(ArgumentResult argResult)
   at System.CommandLine.CliArgument`1.GetDefaultValue(ArgumentResult argumentResult)
   at System.CommandLine.Parsing.ArgumentResult.ValidateAndConvert(Boolean useValidators)
   at System.CommandLine.Parsing.ArgumentResult.GetArgumentConversionResult()
   at System.CommandLine.Parsing.CommandResult.ValidateArguments(Boolean completeValidation)
   at System.CommandLine.Parsing.CommandResult.Validate(Boolean completeValidation)
   at System.CommandLine.Parsing.ParseOperation.Validate()
   at System.CommandLine.Parsing.ParseOperation.Parse()
   at System.CommandLine.Parsing.CliParser.Parse(CliCommand command, IReadOnlyList`1 arguments, String rawInput, CliConfiguration configuration)
   at System.CommandLine.Parsing.CliParser.Parse(CliCommand command, IReadOnlyList`1 args, CliConfiguration configuration)
   at System.CommandLine.CliConfiguration.Parse(IReadOnlyList`1 args)
   at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime, ITelemetry telemetryClient)
   at Microsoft.DotNet.Cli.Program.Main(String[] args)

Further technical details

.NET SDK:
 Version:           10.0.100-preview.4.25177.12
 Commit:            dc1869b111
 Workload version:  10.0.100-manifests.cec059fd
 MSBuild version:   17.15.0-preview-25170-04+88496b9c8

OS: macOS

[KalleOlaviNiemitalo]

The bug is here:

sdk/src/System.CommandLine.StaticCompletions/CompletionsCommand.cs

Lines 70 to 78 in ce2f53b

	var shellName = Path.GetFileName(shellPath);
	if (shellMap.TryGetValue(shellPath, out var shellProvider))
	{
	return shellProvider;
	}
	else
	{
	throw new InvalidOperationException(String.Format(Strings.ShellDiscovery_ShellNotSupported, shellName, string.Join(", ", shellMap.Keys)));
	}

The TryGetValue call should use shellName rather than shellPath.