Don't set security descriptor if the mutex is not owned/new

[jozkee]

Fixes #116014.
Fixes #110454.

[Copilot]

Pull Request Overview

This PR fixes a mutex access control issue by only setting security permissions when the mutex is newly created. The change prevents unnecessary access control modifications on existing mutexes.

• Only sets mutex access control when createdNew is true
• Adds conditional check around the existing security configuration code
• Maintains the same security model but applies it more selectively

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[ericstj]

As discussed - you've done manual testing involving multiple users to ensure this is fixed. I imagine we cannot cover this easily in CI.

[ericstj]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18021213909