Result.LastAgent is *Agent, which holds only unexported fields. The
default json.Marshal renders it as an empty object, which is misleading
when persisted alongside the agent run.
Tag LastAgent as json:"-" and add explicit lowercase JSON tags to the
remaining fields so the serialized shape is stable for callers that
read the persisted result column.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
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