fix(tests): set $commandName in BeforeDiscovery for the now-discovered Context name

The 'Test <_> help parameter help for <commandName>' Context name expands
<commandName> at discovery, but $commandName was only set in BeforeAll. Now that
the prior fix makes this Context actually get discovered, populate $commandName in
BeforeDiscovery too so the generated test names render correctly.

Flagged by Copilot on #35.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tablackburn

tests/Help.tests.ps1

@@ -115,6 +115,7 @@ Describe "Test help for <_.Name>" -ForEach $commands {
         # below; they must also exist here because the nested Context blocks use them in
         # -ForEach, which Pester evaluates during discovery (before BeforeAll runs).
         $command               = $_
+        $commandName           = $command.Name
         $commandHelp           = Get-Help -Name $command.Name -ErrorAction 'SilentlyContinue'
         $commandParameters     = global:FilterOutCommonParameters -Parameters $command.ParameterSets.Parameters
         $commandParameterNames = $commandParameters.Name