Use endpoint for system Nexus detection

[tconley1428]

Summary

• Detect system Nexus schedule payloads by the Temporal system endpoint instead of service/operation registry membership
• Decode system Nexus envelopes from their protobuf payload metadata before nested payload visitation
• Add focused visitor tests for system and non-system endpoint classification

Validation

• uv run ruff check tests/nexus/test_temporal_system_nexus.py temporalio/nexus/system/init.py temporalio/worker/_workflow_instance.py temporalio/bridge/_visitor.py temporalio/nexus/system/_payload_visitor.py
• uv run pytest tests/nexus/test_temporal_system_nexus.py -q
• uv run pyright tests/nexus/test_temporal_system_nexus.py temporalio/nexus/system/init.py temporalio/worker/_workflow_instance.py