Add WorkerPool constructor overload that accepts DiscoverySubscriberLike to support external-only discovery patterns

[conradbzura]

Description

Extend WorkerPool to accept DiscoverySubscriberLike in addition to DiscoveryLike for the discovery parameter. When only a subscriber is provided (no publisher), the pool operates in external mode only — it discovers externally managed workers but cannot spawn or publish its own. If size > 0 is requested with a subscriber-only discovery, the pool raises a clear error explaining that ephemeral workers require a discovery backend with a publisher.

# Full discovery — supports ephemeral, external, and hybrid modes (unchanged)
async with WorkerPool(size=4, discovery=SomeFullDiscovery()):
    ...

# Subscriber-only — external mode only
async with WorkerPool(discovery=some_discovery.subscriber):
    ...

# Subscriber-only + size > 0 — raises ValueError at construction
async with WorkerPool(size=4, discovery=some_discovery.subscriber):
    ...  # ValueError: ephemeral workers require a discovery backend with a publisher

Motivation

Some discovery backends are inherently observe-only. For example, a Docker discovery subscriber watching Swarm-managed containers can detect workers as they appear and disappear, but has no meaningful publish step — the Swarm scheduler owns container lifecycle and labeling. Forcing these backends to implement a no-op publisher to satisfy the DiscoveryLike protocol is unnecessary ceremony. Accepting a bare DiscoverySubscriberLike makes external-only backends a first-class concept and lets the pool validate compatibility at construction time rather than failing silently at runtime.

This change is fully backwards compatible — existing code passes DiscoveryLike instances which satisfy both paths.

Expected outcome

• The discovery parameter on WorkerPool accepts DiscoveryLike | DiscoverySubscriberLike.
• When a DiscoverySubscriberLike without a publisher is provided, the pool operates in external mode: it subscribes to discover existing workers but does not publish.
• When size > 0 is combined with a subscriber-only discovery, the pool raises ValueError with a clear message at construction time.
• No changes to existing DiscoveryLike implementations or callers.