fix: include assignedWorker in session create/get response

[gergesh]

Issue: POST /api/sessions (and GET /api/sessions/{name}) don't return assignedWorker, even though SessionInfo DTO already declares it and GET /api/sessions (list) does return it.
Root cause: getSessions() in manager.core.ts never sets assignedWorker. The plus edition's listSessions() adds it after the fact from the worker repository, but getSessionInfo() doesn't.

Fix — two files:

1. src/core/manager.core.ts — add assignedWorker: this.workerId to both return paths in getSessions() (STOPPED path and active path). (Already patched in this PR.)
2. src/plus/manager.plus.ts — in getSessionInfo(), add assignedWorker: this.workerId to the return object. It currently returns { ...session, engine } — change it to { ...session, engine, assignedWorker: this.workerId }.