fix: handle all SDK stale session error variants

The stale session detection only caught "No message found with
message.uuid" but missed three other error patterns from the SDK:
- "No conversation found with session ID"
- "No conversation found to continue"
- "No conversations found to resume"

These all indicate the upstream session was evicted or lost. Meridian
now catches all four variants and retries with a fresh session instead
of surfacing the error to the user.

Closes #333