Add Temporal Nexus Operation Handler

[Quinn-With-Two-Ns]

Add TemporalOperationHandler for generic Nexus operations

Summary

• Adds TemporalOperationHandler, a generic OperationHandler implementation that maps Nexus operations to Temporal workflows via a composable StartFunction
• Adds TemporalNexusClient for starting workflows (typed and untyped) from within Nexus operation handlers
• Supports subclassing TemporalOperationHandler to customize cancel behavior by overriding cancelWorkflowRun
• Extracts shared workflow start logic into NexusStartWorkflowHelper, reused by both WorkflowRunOperation and the new handler

Test plan

• GenericHandlerTypedStartWorkflowTest — typed workflow with return value
• GenericHandlerTypedProcTest — typed void workflow
• GenericHandlerUntypedStartWorkflowTest — untyped workflow start
• GenericHandlerSyncResultTest — synchronous result path
• GenericHandlerCancelTest — cancel with default and custom behavior

---

Note

Medium Risk
Adds new Nexus/Temporal integration surface area and changes operation-token parsing/validation used for cancel dispatch; regressions could break async operation cancellation or link attachment, though changes are scoped and covered by new tests.

Overview
Adds a new experimental TemporalOperationHandler API for implementing generic Nexus OperationHandlers backed by Temporal, supporting both synchronous results (TemporalOperationResult.sync) and async workflow-start results via operation tokens.

Introduces TemporalNexusClient (+ impl) to start typed/untyped workflows (0–5 args, returning or void) from within Nexus handlers, and routes all workflow-starts through a new shared NexusStartWorkflowHelper that also attaches workflow links to the OperationContext.

Generalizes operation token handling by renaming WorkflowRunOperationToken to OperationToken, adding OperationTokenUtil.loadOperationToken for type-agnostic parsing (used for cancel dispatch), and updating WorkflowRunOperationImpl plus tests (including new coverage for generic handler start/sync/cancel flows).

^{Reviewed by Cursor Bugbot for commit ee62ae7. Bugbot is set up for automated code reviews on this repo. Configure here.}

[Quinn-With-Two-Ns]

@maciejdudko @GregoryTravis This is one issue I found with the approach, due to ambiguity in the lambda we have to return null here for workflows that don't return anything.

[cursor]

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

^{Reviewed by Cursor Bugbot for commit ee62ae7. Configure here.}