Expose Nexus Endpoint in a Nexus Operation Handler

[Quinn-With-Two-Ns]

What was changed

Expose the Nexus Endpoint to the SDK.

Why?

So the handler knows what endpoint the request originally came from for visibility and per endpoint encryption

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

closes #642

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Note

Medium Risk
Adds a new endpoint field to the Nexus bridge task proto and threads it through Nexus worker execution context; while additive, it touches the poll/dispatch path and generated proto code, so regressions could affect Nexus task handling.

Overview
Exposes the Nexus endpoint name to operation handlers by adding Endpoint to NexusOperationInfo and populating it in NexusWorker when creating NexusOperationExecutionContext for both start and cancel requests.

Updates the bridge NexusTask proto (Nexus.cs) to carry an endpoint string alongside the polled task, and adds a unit test asserting NexusOperationExecutionContext.Current.Info.Endpoint matches the invoked endpoint.

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