Log actor exit phase

[rdettai]

Description

Reorganize the actor exit logs:

• explicit where the exit is coming from (initialization? hander? ...)
• remove actor-failure logs (redundant with actor-exit)
• remove redundant ?exit_status and cause?err as both contain the error for the ActorExitStatus::Failure(err) variant

How was this PR tested?

Example log lines:

2025-05-21T12:21:09.001Z  INFO quickwit_actors::spawn_builder: actor-exit actor_id="SourceActor-morning-3QVF" phase=running exit_status=Killed

2025-05-21T12:21:09.035Z  INFO quickwit_actors::spawn_builder: actor-exit actor_id="Indexer-blue-M2W5" phase=handling(quickwit_actors::command::Command) exit_status=Success