Prior to this commit, AbstractBrokerMessageHandler logged the
BrokerAvailabilityEvent object directly at INFO level. With a structured
JSON logging layout, the event could be serialized as an object rather
than via toString(), causing the layout to traverse the event source
(a SimpleBrokerMessageHandler) object graph. That graph contains a
cyclic reference through the client inbound channel executor's thread
factory, which fails serialization at the maximum nesting depth.
This commit logs the event's toString() representation instead, keeping
the same operational signal while preventing structured logging layouts
from traversing framework internals.
Signed-off-by: kodacme <kodac.saito@kodac.me>
