When creating a cron job via `cron add`, if no session_key was provided
(neither via --session-key flag nor CC_SESSION_KEY env), the job was
created with an empty session_key. At runtime, ExecuteCronJob would fail
silently because it couldn't determine the target platform from an empty
session key.
Add server-side validation and auto-detection in handleCronAdd:
- If session_key is empty, attempt to auto-detect from the engine's
active interactive sessions (works when exactly one session exists)
- If auto-detection fails, return a clear error to the client
Add Engine.ActiveSessionKeys() helper method to expose active session
keys for the auto-detection logic. Uses platform != nil check (instead
of agent session alive check) so sessions are discoverable even before
an agent process is started.
Sean
bbd5d56197