Add test for subtitle field parsing in feed and items

- Add test input file (subtitle.rss) with subtitle fields at both feed and item levels
- Add expected output file (subtitle.json) with parsed subtitle values
- Add test case in parser.js to verify subtitle field parsing

Author: openhands-agent

test/input/subtitle.rss

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0">
+  <channel>
+    <title>Example Blog</title>
+    <link>https://example.com/blog</link>
+    <description>A blog about technology and programming</description>
+    <subtitle>Tech insights and tutorials</subtitle>
+    <language>en-us</language>
+    <lastBuildDate>Mon, 15 Jan 2024 10:00:00 GMT</lastBuildDate>
+    <item>
+      <title>First Post</title>
+      <subtitle>An introduction to our blog</subtitle>
+      <link>https://example.com/blog/post-1</link>
+      <description>Welcome to our blog!</description>
+      <pubDate>Mon, 15 Jan 2024 10:00:00 GMT</pubDate>
+      <guid>https://example.com/blog/post-1</guid>
+    </item>
+    <item>
+      <title>Second Post</title>
+      <subtitle>More content for readers</subtitle>
+      <link>https://example.com/blog/post-2</link>
+      <description>Here is another post.</description>
+      <pubDate>Sun, 14 Jan 2024 09:00:00 GMT</pubDate>
+      <guid>https://example.com/blog/post-2</guid>
+    </item>
+  </channel>
+</rss>

test/output/subtitle.json

@@ -0,0 +1,32 @@
+{
+  "feed": {
+    "items": [
+      {
+        "title": "First Post",
+        "subtitle": "An introduction to our blog",
+        "link": "https://example.com/blog/post-1",
+        "pubDate": "Mon, 15 Jan 2024 10:00:00 GMT",
+        "content": "Welcome to our blog!",
+        "contentSnippet": "Welcome to our blog!",
+        "guid": "https://example.com/blog/post-1",
+        "isoDate": "2024-01-15T10:00:00.000Z"
+      },
+      {
+        "title": "Second Post",
+        "subtitle": "More content for readers",
+        "link": "https://example.com/blog/post-2",
+        "pubDate": "Sun, 14 Jan 2024 09:00:00 GMT",
+        "content": "Here is another post.",
+        "contentSnippet": "Here is another post.",
+        "guid": "https://example.com/blog/post-2",
+        "isoDate": "2024-01-14T09:00:00.000Z"
+      }
+    ],
+    "title": "Example Blog",
+    "description": "A blog about technology and programming",
+    "subtitle": "Tech insights and tutorials",
+    "link": "https://example.com/blog",
+    "language": "en-us",
+    "lastBuildDate": "Mon, 15 Jan 2024 10:00:00 GMT"
+  }
+}

test/parser.js

@@ -284,4 +284,8 @@ describe('Parser', function() {
   it('should parse atom:link pagination links', function (done) {
     testParseForFile('pagination-links', 'rss', done);
   });
+
+  it('should parse subtitle field in feed and items', function(done) {
+    testParseForFile('subtitle', 'rss', done);
+  });
 })