Get Run Artifact

Auth-gated mirror of GET /api/agent-chat/artifacts/:short_id.

Supports the same ?offset=&limit= pagination as the prod endpoint — the shared ArtifactViewer pages large tabular artifacts (e.g. 20k people rows) and POSTs the page through this URL.

Reads from the parquet/markdown file referenced by artifact.path when present (post-898f9be8 file-based artifacts) and falls back to the legacy data JSONB for rows written before the refactor. Without this dispatch, every file-backed artifact in the playground viewer came back with data=[] and rendered as "No data".

We don't try to enforce session→artifact ownership here because the short_id is already an unguessable random string and the call is gated behind the playground auth dependency. Anyone with both the URL secret AND a valid Explee token can read; that's the whole privacy model for this surface.