fix: prevent leading slash in S3 keys when prefix is empty (#1863)

[mango766]

Summary

Fix S3SessionManager._get_session_path() producing S3 keys with a leading slash when prefix="" (the default), causing session restore to silently fail on MinIO and S3-compatible backends.

Root cause

# Before: prefix="" produces "/session_<id>/..."
return f"{self.prefix}/{SESSION_PREFIX}{session_id}/"

MinIO strips the leading / on write, storing session_<id>/..., but reads use /session_<id>/... which never matches. read_agent() returns None, initialize() takes the new-session branch, and the agent starts fresh every time.

Fix

session_key = f"{SESSION_PREFIX}{session_id}/"
if self.prefix:
    return f"{self.prefix}/{session_key}"
return session_key

Only prepend prefix/ when prefix is non-empty. All downstream methods (_get_agent_path, _get_message_path) call _get_session_path, so they are all fixed.

Fixes #1863

Test plan

• S3SessionManager(prefix="") → keys have no leading slash
• S3SessionManager(prefix="my-prefix") → keys are my-prefix/session_<id>/...
• Session restore works on MinIO with empty prefix