test(jspg): rename items fixture to array and resolve unused HashMap warning

- Rename fixtures/items.json to fixtures/array.json to better reflect array testing constraints.
- Update reference paths in src/tests/fixtures.rs and across other fixture JSON files.
- Remove unused HashMap import in src/validator/rules/dict.rs to resolve the compiler warning.

src/validator/rules/dict.rs

@@ -1,4 +1,4 @@
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 
 use serde_json::Value;