Add Temporal Nexus Operation Handler

[Quinn-With-Two-Ns]

Add TemporalOperationHandler for generic Nexus operations

Goal

Provide a new base to make it easy to expose more Temporal actions as 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

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Note

Medium Risk
Adds new Nexus-to-Temporal dispatch APIs (start/cancel) and refactors workflow-start/link attachment logic, which could affect Nexus operation execution and cancellation paths. Behavior is guarded by new token validation and single-async-start enforcement but still touches core operation flow.

Overview
Adds an experimental TemporalOperationHandler that maps Nexus start to either a synchronous result or an async workflow-run token via a new TemporalNexusClient (typed/untyped startWorkflow overloads) and TemporalOperationResult.

Refactors workflow-start behavior into NexusStartWorkflowHelper so both the existing WorkflowRunOperationImpl and the new client share the same request construction and workflow-link attachment logic, and enforces only one async start per handler invocation.

Renames/expands operation-token handling (WorkflowRunOperationToken -> OperationToken) and updates OperationTokenUtil to support type-agnostic token loading for cancel dispatch, with updated/added tests covering sync results, typed/untyped starts, double-start errors, and cancel behavior.

^{Reviewed by Cursor Bugbot for commit d448f63. Bugbot is set up for automated code reviews on this repo. Configure here.}

[VegetarianOrc]

Forgive my lack of Java concurrency know how, but could this flag cause a race if you call startWorkflow concurrently? Both attempt to start, race past the check and successfully start a workflow.

[cursor]

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

There are 2 total unresolved issues (including 1 from previous review).

^{Reviewed by Cursor Bugbot for commit f2c65fc. Configure here.}