Add Standalone Activities support to Temporal Nexus Operation Handler

[Quinn-With-Two-Ns]

What was changed

Add Standalone Activities support to Temporal Nexus Operation Handler

Why?

Allow calling Standalone Activities as a Nexus Operation

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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Note

Medium Risk
Touches experimental Nexus operation lifecycle (async tokens, callbacks, cancel routing) and standalone activity RPC fields; behavior is covered by new unit and worker integration tests but spans client and handler paths.

Overview
Adds standalone activity backing for experimental Temporal Nexus TemporalOperationHandler, so handlers can return an async activity-execution operation token (type t=2) instead of only workflow-run tokens.

ITemporalNexusClient gains StartActivityAsync overloads (expression and by name) that schedule activities via NexusActivityStartHelper: default task queue from the operation, deterministic activity Id, Nexus request ID, inbound links, completion callbacks with Nexus-Operation-Token, and UseExisting on-conflict attach options. StartActivityOptions picks up internal links, callbacks, on-conflict, and request ID wired through StartActivityExecution.

Cancel on TemporalOperationHandler peeks token type with LoadTokenType, then cancels the workflow or standalone activity (overridable CancelActivityExecutionAsync). Activity and workflow proto ↔ Nexus link conversion is extended; workflow/activity starts from Nexus now add outbound links from the server response (workflow falls back to a synthetic started-event link). Shared start plumbing moves to NexusOperationStartCommon; workflow start docs note TaskQueue can default to the operation queue.

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