send_to_session was the only agent tool that didn't check session
ownership — an agent acting for user A could read from and write
into user B's session on a multi-user instance.
Add owner parameter and reject access when the target session
belongs to a different user, matching the pattern used by
create_session, list_sessions, and manage_session.
Fixes#1616
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>