test: run integration suite against Cloud

[tconley1428]

What changed

• Add opt-in envconfig support to the shared pytest workflow environment.
• Add WorkflowEnvironment.connect_client so secondary clients retain Cloud credentials.
• Run the normal integration suite in the trusted Cloud CI job and retain the Cloud Operations API check.
• Mark the TLS-mutating client-plugin test local-only.

Validation

• poe lint
• Focused migrated integration tests (8 passed).
• The payload-warning test failed identically after temporarily restoring its original direct connection, so that assertion is unrelated to this change.