Synchronize and reset the DVT middleware connection flag

[damilolaedwards]

Summary

connectedToDVTMiddleware was written from checkDVT without holding any lock, while Proposal and BlindedProposal read it unlocked to decide whether to skip the RANDAO reveal check. It was also never cleared once set, so a client that connected through DVT middleware once kept skipping that check for the rest of its life, even after reconnecting to a plain node later on.

This guards the flag with the existing connection mutex, adds an accessor for the read sites to use, and resets the flag whenever the connection goes inactive so it gets re-evaluated fresh on every reconnect.

Test plan

• New test confirming the flag sets on a Charon-flagged node, clears once the connection goes inactive, and stays clear after reconnecting to a plain node
• New test exercising concurrent connection-state transitions against concurrent flag reads under the race detector, no race reported
• Full repo build and vet pass, all packages still compile