Add benign_application_error sample (BENIGN vs non-BENIGN severity)

[deepika-awasthi]

Adds benign_application_error sample (worker, starter, activity, workflow, README).

Demonstrates ApplicationError(category=ApplicationErrorCategory.BENIGN):

BENIGN → activity failure logged at DEBUG, no activity-failure metrics

Non-BENIGN → WARN log, metrics emitted

Includes JSON logging + SDK log forwarding via LogForwardingConfig.