Fix System Nexus tracing headers

[tconley1428]

Summary

• add a dedicated workflow outbound interceptor path for System Nexus operations
• propagate tracing context through Temporal request headers instead of Nexus transport headers
• add migration guidance for custom workflow interceptors

Validation

• poe test -s tests/contrib/opentelemetry/test_opentelemetry_plugin.py -k signal_with_start
• poe test -s tests/nexus/test_temporal_system_nexus.py -k 'external_workflow_handle_signal_with_start or headers_are_ignored'
• poe lint