Handle transient shared-memory directory state during named mutex open/create

[Copilot]

MutexTests.CrossProcess_NamedMutex_ConcurrentCreateOrOpen was failing in CI on Unix when one process observed /tmp/.dotnet*/shm between directory creation and final ownership/permission normalization. This change makes named mutex shared-memory directory validation tolerate that transient state instead of failing the open/create path.

• Directory race handling

  • Extend SharedMemoryHelpers.EnsureDirectoryExists with the same short transient retry model already used for shared-memory files.
  • Retry when stat/validation sees a directory before the creating process has finished applying the expected owner or mode.

• Failure modes addressed

  • User-scoped directories temporarily reported as owned by the wrong UID.
  • Non-user-scoped directories temporarily reported without the expected owner permissions.

Fixes #130056

[jkotas]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkotas]

/ba-g infrastructure timeouts, no failures in extra platforms related to the fix (too many failures in extra platforms for BA to analyze)

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @VSadov
See info in area-owners.md if you want to be subscribed.