[cachebag]

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After NetworkManager restarts, nmrs emits NetworkManagerRestarted; this PR sends a re-registration request to the existing secret-agent task, which calls SecretAgentHandle::reregister(). When NetworkManager returns, it registers the same agent again, so VPN credential prompts work without restarting the panel.

An issue has been filed to add opt-in automatic secret-agent re-registration after NetworkManager restarts. freedesktop-rs/nmrs#500

Closes #1477