Port system Nexus payload handling to WIT generation

[tconley1428]

Summary

• update system Nexus WIT inputs and regenerate workflow service bindings with nex-gen
• add system Nexus payload visitor support for nested payload encoding/external storage
• export generated system workflow helpers from temporalio.workflow via generation step
• port Temporal system Nexus tests for proto conversion and signal-with-start payload handling

Validation

• poe lint
• uv run -m pytest tests/nexus/test_temporal_system_nexus.py tests/worker/test_visitor.py tests/worker/test_command_aware_visitor.py -q