Unwrap System Nexus Operations in event history

[spkane31]

What was changed

• workflow show (table and --detailed modes) now displays the actual operation name instead of the generic NexusOperationScheduled/NexusOperationCompleted event type when the Nexus endpoint is __temporal_system.
• workflow describe no longer includes pending Nexus operations on the __temporal_system endpoint in the "Pending Nexus Operations" list or count.

Why?

System Nexus operations (endpoint __temporal_system) are implementation details of high-level SDK operations like SignalWithStartWorkflowExecution. Surfacing them as NexusOperationScheduled/NexusOperationCompleted events is confusing. By unwrapping the operation name, the history and describe output reflect what the workflow actually did at a semantic level. Similarly, pending system operations in describe are noise the user has no actionable interest in.

Checklist

1. Closes NA

2. How was this tested:

• Unit tests added in commands.workflow_show_test.go covering all six NexusOperation event types, the non-system-endpoint no-op path, and the missing-scheduled-event (reverse traversal / orphaned completion) path.
• Manual validation: ran temporal workflow show -w <id> against a live workflow that executes a SignalWithStartWorkflowExecution system nexus operation and confirmed events 5 and 6 now show as SignalWithStartWorkflowExecutionScheduled and SignalWithStartWorkflowExecutionCompleted.

4. Any docs updates needed? No