Load a file by path, checking for a session draft first.

If a session draft exists for this notebook:

• Returns the draft state instead of the canonical file.
• If the draft's canonical base revision has drifted, includes a draft_conflict field so the frontend can prompt the user.

Non-notebook files bypass draft logic entirely.

Returns:

• .ipynb -> JSON: { path, cells, metadata, draft_revision?, draft_conflict? }
• text -> raw UTF-8 text (text/plain or application/json)
• binary -> raw bytes (image/*, application/pdf, etc.)