Add WaitGroup for graceful shutdown

[fweilun]

Changes:

main.go

• Add sync.WaitGroup for graceful shutdown.
• Handle SIGINT / SIGTERM to trigger graceful shutdown for all components.

eventserver.go

• Remove internal signal handling
• Pass a stop channel into InitServer so lifecycle is controlled by the caller.
• Flush pending events and stop background routines when shutdown is triggered.

Related issue number

#4392

Testing Strategy

• tested using:

./output/bin/collector \
  --role=Head \
  --runtime-class-name=localtest \
  --ray-cluster-name=my-cluster \
  --ray-root-dir=/tmp/ray-logs

Graceful shutdown was verified by sending SIGINT (Ctrl+C).

[fweilun]

@Future-Outlier could you please take a look at this PR, Thanks.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

[Future-Outlier]

Thanks for the PR!

The collector.go still has its own signal.Notify which conflicts with main.go. Please follow the graceful shutdown pattern in historyserver/pkg/historyserver/server.go - it only listens to the stop channel without any signal handling.

[Future-Outlier]

cc @machichima @win5923 to help review