fix(nexus): clear operation callbacks on 'abandon' cancellation.

[cconstable]

Was seeing a flakey test (TestAsyncOperationFromWorkflow_CancellationTypes/Abandon) and traced it back to a bug.

What was changed

• Cancelling withNexusOperationCancellationTypeAbandon now correctly clears nexus callbacks.
• Added a small helper for getting the nexus operation command.

Why?

When you cancel a workflow running a nexus operation with NexusOperationCancellationTypeAbandon, it operations futures directly but doesn't clear the startedCallback or completedCallback. Then, when a NexusOperationStarted (or completed) event is received, the stale callbacks fires and attempts to set an already resolved future which panics, which WFT fails.

Fixes #2495

Checklist

• Added tests to verify that we clear callbacks.

[cconstable]

added a small helper since this manual formatting was repeated in a number of places.

[cconstable]

the fix. clears out the callbacks so we don't try to resolve an already resolved future.

[Quinn-With-Two-Ns]

We have the same problem for Child Workflows that already does handle this, is there a reason you solved this in a different way then Child Workflows?

sdk-go/internal/internal_event_handlers.go

Line 1846 in 9a42fac

if childWorkflow.handled {

[cconstable]

@Quinn-With-Two-Ns I matched the existing nexus pattern of nilling out the callbacks:

func (wc *workflowEnvironmentImpl) RequestCancelNexusOperation(seq int64) {
    ...
    data.startedCallback = nil
}

The handled flag in child workflows looks like it means we hit a terminal state and we can skip any extra completions. I don't think this maps to nexus directly since nexus operations can complete without a started event. So I think the options are:

1. follow the existing nexus pattern of nilling out the callbacks (what is in the PR)
2. add a handled flag and some new logic to all the nexus lifecycle handlers to nil out the started callbacks after firing it (seems like too much scope for this PR)
3. add a new abandoned flag and use it for this case to skip completion callbacks but now this nexus lifecycle handler will diverge from what the others are doing

I actually prefer the idea of having flags/enums for the state here but I feel like that should be it's own PR and would probably need more discussion. I could open another to make that change.