[WIP,DO_NOT_REVIEW] System Nexus Endpoint for cross-workflow operations

[yiminc]

Summary

Introduces a System Nexus Endpoint feature that enables workflows to invoke built-in Temporal server operations using the existing Nexus infrastructure.

What's included:

• Design document (docs/architecture/system-nexus-endpoint.md)
• Implementation plan (docs/architecture/system-nexus-endpoint-implementation-plan.md)
• Initial implementation of WaitExternalWorkflowCompletion operation
• Integration tests

Motivation

Instead of adding new commands and history events for each new workflow capability (like waiting for external workflow completion - #680), we leverage the Nexus operation model to extend workflow functionality in a uniform, extensible manner.

Key Design Points

• No user-defined endpoint required - workflows specify __temporal_system endpoint
• Server-side handling - requests route directly to History service via callback mechanism
• Reuses existing infrastructure - Nexus operation state machines, outbound queues, callbacks, and events
• Extensible - new system operations can be added without new command types
• Namespace isolation - system operations only work within the same namespace

Test plan

• Unit tests for system nexus operation handlers
• Integration tests for WaitExternalWorkflowCompletion scenarios
  • Wait for already completed workflow
  • Wait for running workflow to complete
  • Wait for workflow that fails

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