AgentClient now verifies the worker's TLS cert fingerprint against
SwarmHost.client_cert_fingerprint at __aenter__ time, on top of CA
validation. Required before fanning master-orchestrated topology
deploys out across multiple swarm hosts: CA pinning alone allows any
cert signed by the master CA, which is too coarse once a single
deploy can target N hosts.
Mismatch raises FingerprintMismatchError so callers can distinguish
"wrong worker on the wire" from a transport hiccup.
anti
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