fix: return 404 for deleted activity in activity_retrieve endpoint

[sricharan-varanasi]

Changes

• Added null check after ActivitiesCRUD.get_by_id() - raises ActivityDoeNotExist (404) when activity is not found
• Updated test assertion to expect HTTP 404 instead of != 200

Why

Previously, fetching a deleted activity without a version would cause a NullPointerError or undefined behavior downstream (attempting schema.applet_id on None). Now it returns a proper 404.

Notes

• ActivityDoeNotExist already existed in apps/activities/errors.py
• Only affects the unversioned code path in activity_retrieve (the versioned path was already safe)

[divbzero]

@sricharan-varanasi The fix looks good and works on my machine. Thank you!