Extracts book_parser.py from book_handler.py

Extracts out a BaseBookParser, PDFParser and MarkdownParser class from book_handler logic

The basic contract is that the parser classes populate 4 attributes that are used by the book_handler logic
- content_texts
- content_lengths
- book_metatdata
- processed_nav_structures (used by the method get_chapters on the BaseBookParser class)

Adds tests to validate the changes as well in tests/
Run tests from the abogen dir as follows

```bash
python -m unittest discover
```

tests/test_epub_html_nav_parsing.py

@@ -0,0 +1,184 @@
+import unittest
+import os
+import shutil
+import sys
+from ebooklib import epub
+
+# Ensure import path
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
+
+from abogen.book_parser import get_book_parser
+
+class TestEpubHtmlNavParsing(unittest.TestCase):
+    """
+    Tests for EPUB 3 HTML5 Navigation Document parsing logic (_parse_html_nav_li).
+    """
+
+    def setUp(self):
+        self.test_dir = "tests/test_data_nav"
+        if os.path.exists(self.test_dir):
+            shutil.rmtree(self.test_dir)
+        os.makedirs(self.test_dir)
+        self.epub_path = os.path.join(self.test_dir, "nav_test.epub")
+
+    def tearDown(self):
+        if os.path.exists(self.test_dir):
+            shutil.rmtree(self.test_dir)
+
+    def _create_epub_with_custom_nav(self, nav_html_content):
+        """
+        Creates an EPUB with a manually injected HTML Navigation Document.
+        """
+        book = epub.EpubBook()
+        book.set_identifier("navtest123")
+        book.set_title("Nav Test Book")
+        
+        # Add some content files
+        c1 = epub.EpubHtml(title="Chapter 1", file_name="chap1.xhtml", lang="en")
+        c1.content = "<h1>Chapter 1</h1><p>Text 1</p>"
+        book.add_item(c1)
+        
+        c2 = epub.EpubHtml(title="Chapter 2", file_name="chap2.xhtml", lang="en")
+        c2.content = "<h1>Chapter 2</h1><p>Text 2</p>"
+        book.add_item(c2)
+
+        # Create the Nav item manually to control the HTML structure exactly
+        # Use EpubHtml + OPF patching because EpubNav forces auto-generation
+        nav = epub.EpubHtml(title="Nav", file_name="nav.xhtml")
+        nav.content = nav_html_content
+        book.add_item(nav)
+        
+        # We must set spine manually
+        book.spine = [nav, c1, c2]
+        
+        epub.write_epub(self.epub_path, book)
+        
+        # Patch the OPF to remove toc="ncx" default which causes crash 
+        # because we intentionally excluded the legacy NCX file.
+        import zipfile
+        
+        with zipfile.ZipFile(self.epub_path, 'r') as zin:
+            opf_content = zin.read('EPUB/content.opf').decode('utf-8')
+            opf_content = opf_content.replace('toc="ncx"', '')
+            
+            # Repack
+            TEMP_EPUB = self.epub_path + ".temp"
+            with zipfile.ZipFile(TEMP_EPUB, 'w') as zout:
+                for item in zin.infolist():
+                    if item.filename == 'EPUB/content.opf':
+                        zout.writestr(item, opf_content)
+                    else:
+                        zout.writestr(item, zin.read(item.filename))
+        
+        shutil.move(TEMP_EPUB, self.epub_path)
+
+    def test_basic_html_nav_parsing(self):
+        """
+        Test parsing of a standard flat list of links.
+        """
+        nav_html = """
+        <nav epub:type="toc" id="toc">
+            <h1>Table of Contents</h1>
+            <ol>
+                <li><a href="chap1.xhtml">Chapter 1</a></li>
+                <li><a href="chap2.xhtml">Chapter 2</a></li>
+            </ol>
+        </nav>
+        """
+        self._create_epub_with_custom_nav(nav_html)
+        
+        parser = get_book_parser(self.epub_path)
+        parser.process_content()
+        chapters = parser.get_chapters()
+        
+        # Filter out "Nav" or "Introduction" prefix content found from the Nav file itself
+        chapters = [c for c in chapters if "Chapter" in c[1] or "Section" in c[1]]
+        
+        self.assertEqual(len(chapters), 2)
+        self.assertEqual(chapters[0][1], "Chapter 1")
+        self.assertEqual(chapters[1][1], "Chapter 2")
+
+    def test_nested_html_nav_parsing(self):
+        """
+        Test parsing of nested lists (Sub-chapters).
+        """
+        nav_html = """
+        <nav epub:type="toc">
+            <ol>
+                <li>
+                    <a href="chap1.xhtml">Chapter 1</a>
+                    <ol>
+                        <li><a href="chap2.xhtml">Section 1.1</a></li>
+                    </ol>
+                </li>
+            </ol>
+        </nav>
+        """
+        # Note: In this test setup, chap2 is serving as "Section 1.1" effectively
+        self._create_epub_with_custom_nav(nav_html)
+        
+        parser = get_book_parser(self.epub_path)
+        parser.process_content()
+        chapters = parser.get_chapters()
+        
+        ids = [c[1] for c in chapters]
+        self.assertIn("Chapter 1", ids)
+        self.assertIn("Section 1.1", ids)
+
+    def test_span_header_parsing(self):
+        """
+        Test parsing of <li><span>Header</span><ol>...</ol></li> pattern.
+        This represents a grouping header that isn't a link itself.
+        """
+        nav_html = """
+        <nav epub:type="toc">
+            <ol>
+                <li>
+                    <span>Part I</span>
+                    <ol>
+                        <li><a href="chap1.xhtml">Chapter 1</a></li>
+                    </ol>
+                </li>
+            </ol>
+        </nav>
+        """
+        self._create_epub_with_custom_nav(nav_html)
+        
+        parser = get_book_parser(self.epub_path)
+        parser.process_content()
+        
+        chapters = parser.get_chapters()
+        chapter_titles = [c[1] for c in chapters]
+        
+        self.assertIn("Chapter 1", chapter_titles)
+        self.assertNotIn("Part I", chapter_titles) 
+        
+        # Check internal structure
+        # Find the node named "Part I" in the processed structure
+        root_node = next(node for node in parser.processed_nav_structure if node['title'] == "Part I")
+        
+        self.assertEqual(root_node['title'], "Part I")
+        self.assertFalse(root_node['has_content'])
+        self.assertEqual(len(root_node['children']), 1)
+        self.assertEqual(root_node['children'][0]['title'], "Chapter 1")
+
+
+    def test_identify_nav_item(self):
+        """Test the _identify_nav_item method specifically."""
+        nav_html = """
+        <nav epub:type="toc" id="toc"><h1>TOC</h1><ol><li><a href="c1.html">C1</a></li></ol></nav>
+        """
+        self._create_epub_with_custom_nav(nav_html)
+        parser = get_book_parser(self.epub_path)
+        # Note: _identify_nav_item relies on self.book being loaded
+        # The parser constructor or process_content handles load()
+        # But here we can call load directly if needed, or rely on normal flow up until navigation
+        parser.load()
+        nav_item, nav_type = parser._identify_nav_item()
+        
+        self.assertEqual(nav_type, "html")
+        self.assertIsNotNone(nav_item)
+        self.assertTrue("nav.xhtml" in nav_item.get_name())
+
+if __name__ == "__main__":
+    unittest.main()