chore: getJsonHeaders as util function since reusable

src/utils/json.ts

@@ -0,0 +1,18 @@
+/**
+ * Collects all unique keys from an array of row objects, preserving first-encountered order.
+ * Handles sparse rows where different rows may have different keys.
+ *
+ * @param rows - Array of flattened row objects
+ * @returns Array of unique header strings in insertion order
+ *
+ * @example
+ * getJsonHeaders([{ a: '1' }, { a: '2', b: '3' }]) // → ['a', 'b']
+ */
+export function getJsonHeaders(rows: Record<string, string>[]): string[] {
+  return Array.from(
+    rows.reduce<Set<string>>((set, row) => {
+      Object.keys(row).forEach((key) => set.add(key));
+      return set;
+    }, new Set())
+  );
+}