Expose active workflow instances on WorkflowDefinition

[mcruzdev]

What would you like to be added:

Expose the full set of active (non-terminal) WorkflowInstances tracked by a WorkflowDefinition, similar to the existing scheduledInstances() accessor.

WorkflowDefinition (impl/core/src/main/java/io/serverlessworkflow/impl/WorkflowDefinition.java) already keeps every running instance in an internal map:

private Map<String, WorkflowInstance> activeInstances = new ConcurrentHashMap<>();

but the only public accessor is a single-instance lookup:

public Optional<WorkflowInstance> activeInstance(String instanceId)

There's no way to enumerate all active instances for a definition. Meanwhile, scheduledInstances already has a bulk accessor that returns an unmodifiable view:

public Collection<WorkflowInstance> scheduledInstances() {
    return Collections.unmodifiableCollection(scheduledInstances);
}

I'd like to add an equivalent for active instances, e.g.:

public Collection<WorkflowInstance> activeInstances() {
    return Collections.unmodifiableCollection(activeInstances.values());
}

Why is this needed:

In quarkus-flow we're adding a REST endpoint that lists the in-memory active workflow instances on a runner pod, so a rebalancer service can tell which instances are actively being processed on a given pod vs. only persisted in the DB (see PR #892).

Since the SDK doesn't expose this today, the current implementation duplicates the tracking with a separate ActiveInstanceRegistry that listens to WorkflowExecutionListener lifecycle events and maintains its own copy of the same data WorkflowDefinition already holds internally.

As pointed out in review feedback by @fjtirado, this duplication isn't necessary — WorkflowDefinition is already the source of truth for active instances, it just isn't public. Exposing activeInstances() (mirroring scheduledInstances()) would let quarkus-flow (and other SDK consumers) query this directly instead of re-implementing the same bookkeeping, and let us drop ActiveInstanceRegistry in favor of the new method.