Fix pending Nexus task cleanup

[VegetarianOrc]

What was changed

• Register Nexus operation tasks with the workflow instance to suppress pending-task destruction warnings and ensure normal cleanup during eviction.

Why?

• Worker shutdown while a workflow waits for a Nexus operation can produce Task was destroyed but it is pending! warnings.
• Nexus tasks should follow the same registration and eviction cleanup path as activity and child workflow tasks.

Checklist

1. Closes [Bug] Nexus operation handle task logs "Task was destroyed but it is pending!" on worker shutdown/eviction #1960

2. How was this tested:

• Added integration tests to confirm eviction scenarios occur as expected.

3. Any docs updates needed?

N/A

[PM-JoakimGustavsson]

@VegetarianOrc Thank you so much for looking into this so quickly!