Generic Temporal Nexus Operation Handler

[Quinn-With-Two-Ns]

What was changed

Added a generic Nexus operation handler that consolidates the common pattern of "either return a synchronous
result or start a workflow as an async operation."

Why?

The existing WorkflowRun / SyncOperation split forces users to pick the operation shape up front, which is awkward when:

• The start handler decides at runtime whether to respond sync or async.
• The handler needs access to the Temporal client.
• The user wants a custom cancel implementation (e.g. terminate instead of cancel).
• Difficult to extend as we add new operations

The new TemporalOperation unifies these cases behind one API and makes the common path much shorter.

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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Note

Medium Risk
Changes experimental Nexus operation start/cancel and callback token wiring shared by old and new handlers; mistakes could break async completion or cancel the wrong workflow, though coverage is broad.

Overview
Adds a unified experimental Nexus API (TemporalOperationHandler.FromHandleFactory) so one handler can return either a sync result or an async workflow-run token, with ITemporalNexusClient / TemporalNexusClient for starting workflows and accessing TemporalClient for signals and similar work.

Shared workflow-start behavior (links, completion callbacks, Nexus-Operation-Token header with case-insensitive dedup, UseExisting conflict options, outbound links) moves into NexusWorkflowStartHelper; WorkflowRunOperationContext now delegates there instead of inlining that logic.

Operation tokens gain ParseToken, an OperationToken record, and a WorkflowRunOperationTokenType constant; cancel on the generic handler validates namespace and token type, then calls overridable CancelWorkflowRunAsync (default: cancel workflow by ID). Unit and worker integration tests cover tokens, sync/async paths, cancel, links, and conflict policy.

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[jmaeagle99]

Still reviewing but wanted to leave my initial comments.

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[jmaeagle99]

Do we want to guide customers to use TemporalNexusOperationHandler instead of WorkflowRunOperationHandler? Obsoleting would be a forcing function. If not that, maybe update the comments on WorkflowRunOperationHandler and related types to recommend using the new types.