Fix concurrent-import crash in Run.Parallel (thread-unsafe verb patch)

[nohwnd]

Run.Parallel runs every test file in its own ForEach-Object -Parallel runspace, but those runspaces share one process and the same loaded assemblies. On import Pester calls [Pester.VerbsPatcher]::AllowShouldVerb(...), which reflects into PowerShell's process-global System.Management.Automation.Verbs.s_validVerbs dictionary and does validVerbs["Should"] = true (plus a background task that removes it after 5 seconds).

Dictionary<TKey,TValue> is not thread-safe, so when several workers Import-Module Pester at the same time the concurrent writes tear the internal bucket array and throw:

Exception calling "AllowShouldVerb": "Index was outside the bounds of the array."

That kills the parallel Invoke-Pester, no [Pester.Run] object comes back, and tst/Pester.RSpec.Parallel.ts.ps1 then fails on Windows PS7 with follow-on errors like property TotalCount cannot be found or property Containers cannot be found. It is Windows-only and intermittent, but it is not flakiness, it is an unsynchronized write to a shared dictionary, and it reproduces on main.

The fix is in VerbsPatcher.cs only. Pester.dll loads once per process, so a static lock is shared across all runspaces. I wrap both the insert and the delayed removal in lock (s_lock) and guard them with ContainsKey, so the first import mutates the dictionary and the rest are no-ops. No await inside the lock.

Not Dictionary.TryAdd, it does not exist on the net462 target. Not a bare retry/catch, a torn dictionary is permanently corrupt, not transiently retryable.

Verified: build.ps1 -Clean is clean, test.ps1 -File tst/Pester.RSpec.Parallel.ts.ps1 -SkipPTests passes (22), and 1..8 | ForEach-Object -Parallel { Import-Module ./bin/Pester.psd1 } -ThrottleLimit 8 no longer crashes.