v0.5.0 — Fix container_id decoding for compute sessions

Fixed

• Session.ContainerID and SessionInfo.ContainerID now decode compute (MR)
  sessions correctly. The live API returns container_id as a JSON array for
  compute sessions (absent entirely for VPN sessions), but the field was typed
  as a bare string; any account with an active compute rental hit a decode
  error on the entire /sessions list and /services/:uuid/info, not just
  that one field. Verified against a live account with an active session.

Changed

• Breaking: Session.ContainerID and SessionInfo.ContainerID are now
  FlexString instead of string. FlexString behaves like a normal string
  (.String(), comparable to string literals) but accepts both the legacy
  bare-string and the current array-of-strings shape, joining multiple
  container IDs with ", ". Callers holding these fields in an explicitly
  string-typed variable need .String() or an explicit conversion.