GHSA-rxhg-vcww-2mpw: 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. This closes the injection vector: separators, whitespace, parentheses, and quotes cannot survive sanitization, so an expression-based ORDER BY oracle is not constructable. Affected versions < fleet-v4.89.0. Fixed in fleet-v4.89.0. Credit Thanks to @axel-corsiez for the report.