test: run Nexus tests against Cloud

[tconley1428]

Summary

• Create Nexus endpoints through the Cloud Operations API for Cloud Nexus tests.
• Verify endpoint readiness with a disposable end-to-end Nexus operation, then remove it.
• Delete test endpoints after each test and before Cloud namespace teardown.
• Run Cloud Nexus tests in a dedicated CI job with higher parallelism.

Validation

• poe test -s tests/nexus/test_operation_token.py
• poe test -s --workflow-environment envconfig --collect-only tests/nexus
• poe test -s -k test_nexus_client_updates_when_worker_client_changes tests/nexus/test_nexus_client_updates.py
• poe lint