Add Nexus system payload codec rewrite support

[tconley1428]

Summary

• add operation-aware Nexus payload rewrite handling in workflow completion encoding
• generate and package Temporal Nexus system models under temporalio.nexus.system
• add an end-to-end Python test using actual generated nexus-rpc-gen output

Validation

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