Port system Nexus payload handling to WIT generation

[tconley1428]

Summary

Adds WIT-generated Python bindings for Temporal system Nexus operations, starting with WorkflowService.SignalWithStartWorkflowExecution, and exposes it as workflow.signal_with_start_workflow.

The generated call targets the server-reserved __temporal_system endpoint and uses the fully qualified service name temporal.api.workflowservice.v1.WorkflowService.

What changed

• Adds system Nexus API generation from the WIT model checked into sdk-core.
• Wires system Nexus generation into proto generation.
• Generates the Python workflow service package for SignalWithStartWorkflowExecution.
• Re-exports generated workflow operations from temporalio.workflow so newly generated operations are exposed without manual edits.
• Adds a system Nexus operation registry keyed by fully qualified service and operation name.
• Adds system Nexus payload handling so:
  • the outer system Nexus envelope remains binary proto encoded
  • nested payload-bearing fields still go through normal codec and external storage processing
• Updates payload visitor generation to route recognized system Nexus operation inputs through the generated system payload visitor.

Testing

Adds coverage for:

• binary proto round trips for generated system Nexus request and response envelopes
• workflow.signal_with_start_workflow using the system Nexus path
• codec behavior proving the outer proto envelope is not codec-encoded
• nested payload traversal for workflow inputs, signal inputs, memo, and user metadata