Fleet: ORDER BY column injection on activity list endpoints

### Summary An authenticated user with read access to Activity could influence the `ORDER BY` clause of the activity list endpoints by supplying an arbitrary sort column: - `GET /api/v1/fleet/activities` (`ListActivities`) - `GET /api/v1/fleet/hosts/{id}/activities` (`ListHostPastActivities`) This originated from the deprecated cursor-pagination helper (`appendListOptionsWithCursorToSQL`), which interpolated the caller-supplied order key into SQL without an allowlist. The original report's `node_key` extraction scenario (`/api/v1/fleet/labels/{id}/hosts`) was remediated separately in #44385; these two activity endpoints were the residual call sites, neither of which joins the `hosts` table, so `node_key` was never reachable through them. ### Impact Read-only. Because the order key was interpolated, an authenticated user with Activity read could sort by columns not otherwise returned in these responses. The exposure was bounded to columns on `activity_past` (e.g. `details` on `/api/v1/fleet/activities`, which is not in that endpoint's SELECT; host-only activities are already excluded by `WHERE host_only = false`). There is no privilege escalation, write access, or reachability of `node_key` or other host-join columns through these endpoints. ### Remediation The deprecated helper was removed from the codebase. Both endpoints now pass the caller-supplied sort column through `SanitizeColumn`, which strips all characters except `[\w-.]` and backtick-quotes each identifier segment.