Werk #15658: Agent auto-registration: Re-register if registration at monitoring site is gone

When starting, the agent controller checks if the auto-registration (introduced in werk 15243) is configured. If this is the case, the controller proceeds to register at the configured site, but only if it is not already registered there according to its local configuration.

As of this werk, if the connection to the configured site is found in the local configuration, we additionally check if the configured site also reports that the host is registered. If this is not the case, we proceed with the registration in order to re-establish the connection.

This feature is relevant in scenarios where hosts are temporarily shut down and removed from the monitoring site. Once these hosts re-boot, they will re-register st. they are monitored again.

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