refactor(reference schema): extracted article to separate schema

src/models/articleSchema.ts

+import { z } from 'zod';
+
+export const articleSchema = z.object({
+  title: z.string(),
+  authors: z.array(z.string()),
+  journal: z.string(),
+  year: z.number(),
+  link: z.string(),
+  pubmedId: z.string(),
+  citationCount: z.number(),
+});

src/models/referenceSchema.ts

 import { z } from 'zod';
+import { articleSchema } from './articleSchema';
 
 export const referenceSchema = z.object({
   link: z.string(),
-  article: z
-    .object({
-      title: z.string(),
-      authors: z.array(z.string()),
-      journal: z.string(),
-      year: z.number(),
-      link: z.string(),
-      pubmedId: z.string(),
-      citationCount: z.number(),
-    })
-    .optional(),
+  article: articleSchema.optional(),
   type: z.string(),
   resource: z.string(),
   id: z.number(),