Fix Actor configuration override for custom upgrade paths

[apankov1]

Summary

• Convert Actor.configuration into a real static method so subclasses can reliably override socket settings, and make fetch/getActor pull config from the subclass constructor.
• Add a regression test confirming a custom upgradePath triggers onWebSocketUpgrade and that /ws still falls through to the HTTP handler.

Testing

• npm run test --workspace packages/core
• Validated end-to-end in the project where misrouted WebSocket upgrades originally surfaced.

Code in this PR was AI-assisted and exercised against the pet project that experienced the bug.