The empty Zig anonymous struct `.{}` serializes to `[]` (tuple → JSON
array), not `{}`. The dispatch path's `InputParams.jsonParse` requires
the params field to begin with `object_begin`, so the previous test
fixtures hit `error.UnexpectedToken` → `error.InvalidJSON` instead of
exercising the production code paths.
Switch the two empty-params test cases to raw JSON string literals,
which the testing helper passes through unchanged (string literals are
pointer types and skip `std.json.Stringify.valueAlloc`). The production
code paths under test are unchanged.
53b41966fd