[JoshuaFrenchwood]

What was changed

Nexus operation handlers now propagate their NexusSerializationContext to the workflow updates
they send.

• Workflow updates: the update request carries the context. The worker encodes the update's
  result, failure and rejection with it, and update handles decode with the context returned on the
  update and poll responses.
• Update arguments keep the workflow context.

Why?

A Nexus handler returns the result and failure of the update it sends to the Nexus caller. The
caller decodes them with the operation's NexusSerializationContext, so a codec keyed on the
context couldn't decode them when they were encoded with the workflow context.

Checklist

1. Closes

2. How was this tested:

• Unit tests for the client invoker's update request, completed update and poll.
• WorkflowUpdateNexusSerializationContextTest: the in-memory server sends real updates with the
  context set, covering update result, failure, rejection and replay.
• End-to-end NexusSerializationContextPropagationTest on the dev server: a caller workflow goes
  through a Nexus endpoint to a handler that sends a workflow update.

3. Any docs updates needed?

[Evanthx]

LGTM!