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
Adds new Nexus operation handler and client APIs that affect workflow-start plumbing, token handling, and cancellation behavior; bugs could impact operation cancellation or link/callback propagation.

Overview
Introduces a new generic Temporal-backed Nexus operation handler (TemporalNexusOperationHandler) that lets a start function return either a synchronous value or an async operation token via TemporalOperationResult, and supports overridable cancel behavior for workflow-run tokens.

Adds a Nexus-aware client (ITemporalNexusClient/TemporalNexusClient) plus NexusWorkflowStartHelper to centralize workflow-start plumbing (task queue defaulting, conflict attach options, inbound/outbound links, completion callbacks, and token header injection).

Refactors NexusWorkflowRunHandle token handling to expose ParseToken (validates base64url/JSON/version while leaving token-type enforcement to callers) and updates WorkflowRunOperationContext to use the shared start helper. Expands tests to cover token encoding/parsing, TemporalOperationResult, and end-to-end worker behavior for the new handler (start-by-expression/name, sync results, links, conflict policy, and cancellation/overrides).

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