test: add tests for article metadata extraction

- extractArticleMetadata: 6 tests for title/preview extraction
- mapTweetResult: 3 tests for article field inclusion

tests/extract-article-metadata.test.ts

@@ -0,0 +1,88 @@
+import { describe, expect, it } from 'vitest';
+import type { GraphqlTweetResult } from '../src/lib/twitter-client-types.js';
+import { extractArticleMetadata } from '../src/lib/twitter-client-utils.js';
+
+describe('extractArticleMetadata', () => {
+  it('returns undefined when no result', () => {
+    expect(extractArticleMetadata(undefined)).toBeUndefined();
+  });
+
+  it('returns undefined when no article field', () => {
+    const result: GraphqlTweetResult = {
+      rest_id: '123',
+      legacy: { full_text: 'Hello world' },
+    };
+    expect(extractArticleMetadata(result)).toBeUndefined();
+  });
+
+  it('returns undefined when article has no title', () => {
+    const result: GraphqlTweetResult = {
+      rest_id: '123',
+      article: {
+        article_results: {
+          result: {},
+        },
+      },
+    };
+    expect(extractArticleMetadata(result)).toBeUndefined();
+  });
+
+  it('extracts title from article_results.result', () => {
+    const result: GraphqlTweetResult = {
+      rest_id: '123',
+      article: {
+        article_results: {
+          result: {
+            title: 'Building Context Graphs for GTM',
+          },
+        },
+      },
+    };
+
+    const metadata = extractArticleMetadata(result);
+    expect(metadata).toEqual({
+      title: 'Building Context Graphs for GTM',
+      previewText: undefined,
+    });
+  });
+
+  it('extracts title and preview_text from article_results.result', () => {
+    const result: GraphqlTweetResult = {
+      rest_id: '123',
+      article: {
+        article_results: {
+          result: {
+            title: 'Building Context Graphs for GTM',
+            preview_text:
+              'Foundation Capital recently argued that one of the next trillion-dollar opportunities in AI will come from context graphs.',
+          },
+        },
+      },
+    } as GraphqlTweetResult;
+
+    const metadata = extractArticleMetadata(result);
+    expect(metadata).toEqual({
+      title: 'Building Context Graphs for GTM',
+      previewText:
+        'Foundation Capital recently argued that one of the next trillion-dollar opportunities in AI will come from context graphs.',
+    });
+  });
+
+  it('falls back to article.title when article_results.result.title missing', () => {
+    const result: GraphqlTweetResult = {
+      rest_id: '123',
+      article: {
+        title: 'Fallback Title',
+        article_results: {
+          result: {},
+        },
+      },
+    };
+
+    const metadata = extractArticleMetadata(result);
+    expect(metadata).toEqual({
+      title: 'Fallback Title',
+      previewText: undefined,
+    });
+  });
+});