Iterating a JSON array with GET re-parses the entire string on every
call, so walking N elements is O(N^2). ARRAY_SPLIT parses the array
once and returns its elements as a CMake list, letting each element be
queried individually in linear total time.
Each element is re-serialized as compact JSON and encoded to survive
CMake list parsing: '[' and ']' inside strings are emitted as \u005B
and \u005D, and ';' is escaped as '\;'. Every element therefore stays
valid, re-queryable JSON.
Fixes: #27985
