Add test utility to create Nexus endpoint

[Quinn-With-Two-Ns]

Add test utility to create Nexus endpoint. This matches what we have in Java and Dotnet

---

Note

Adds testing utilities to manage Nexus endpoints directly from WorkflowEnvironment and updates Nexus tests accordingly.

• New WorkflowEnvironment.create_nexus_endpoint and delete_nexus_endpoint using OperatorService (adds temporalio.api.nexus.v1 and operatorservice.v1 imports)
• Nexus tests now call env.create_nexus_endpoint(make_nexus_endpoint_name(task_queue), task_queue) instead of tests.helpers.nexus.create_nexus_endpoint
• _start_wf_and_nexus_op updated to accept env and create the endpoint internally

^{Written by Cursor Bugbot for commit 71c8912. This will update automatically on new commits. Configure here.}