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Prevent DoS and thread starvation in Redfish session token refresh.

RedfishSessionAuthPlugin held the main mutex across synchronous network
calls during token refresh, which could block all proxy threads.

This change separates the network refresh logic from the state mutex using a
dedicated fetch mutex. It also adds a 15-second cooldown and double-checked
locking to prevent repeated forced refreshes from causing thread starvation.
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PiperOrigin-RevId: 971512071
Change-Id: Ieaf8c6d172105d930b8dcd0831bd9e09104d8203