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_book_handler_regression.py

@@ -0,0 +1,84 @@
+import unittest
+import os
+import sys
+import shutil
+import time
+from PyQt6.QtWidgets import QApplication
+
+# Ensure we can import the module
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), '..')))
+
+from abogen.book_handler import HandlerDialog
+from ebooklib import epub
+
+# We need a QApplication instance for QWriter/QDialog
+app = QApplication(sys.argv)
+
+class TestBookHandlerRegression(unittest.TestCase):
+
+    def setUp(self):
+        self.test_dir = "tests/test_data_handler"
+        if os.path.exists(self.test_dir):
+            shutil.rmtree(self.test_dir)
+        os.makedirs(self.test_dir)
+        self.sample_epub_path = os.path.join(self.test_dir, "test_book.epub")
+        self._create_sample_epub()
+
+    def tearDown(self):
+        HandlerDialog.clear_content_cache()
+        if os.path.exists(self.test_dir):
+            shutil.rmtree(self.test_dir)
+
+    def _create_sample_epub(self):
+        book = epub.EpubBook()
+        book.set_identifier("id123456")
+        book.set_title("Sample Book")
+        book.set_language("en")
+
+        c1 = epub.EpubHtml(title="Intro", file_name="intro.xhtml", lang="en")
+        c1.content = "<h1>Introduction</h1><p>Welcome to the book.</p>"
+        book.add_item(c1)
+        book.spine = ["nav", c1]
+        book.add_item(epub.EpubNcx())
+        book.add_item(epub.EpubNav())
+        epub.write_epub(self.sample_epub_path, book)
+
+    def test_handler_initialization(self):
+        """Test that HandlerDialog processes the book correctly."""
+        # HandlerDialog starts processing in a background thread in __init__
+        # We assume headless environment, so we won't show it.
+        # But we need to wait for the thread to finish.
+        
+        dialog = HandlerDialog(self.sample_epub_path)
+        
+        # Wait for thread to finish
+        # The dialog emits no signal publicly, but we can check internal state or thread
+        
+        start_time = time.time()
+        while time.time() - start_time < 5:
+            # HandlerDialog logic:
+            # _loader_thread.finished connect to _on_load_finished
+            # _on_load_finished populates content_texts and content_lengths
+            
+            # We can check if content_texts is populated
+            if dialog.content_texts:
+                break
+            app.processEvents() # Process Qt events to let thread signals propagate
+            time.sleep(0.1)
+            
+        self.assertTrue(len(dialog.content_texts) > 0, "HandlerDialog failed to process content in time")
+        
+        # Validate content similar to what we expect
+        # intro.xhtml should be there
+        found_intro = False
+        for key, text in dialog.content_texts.items():
+            if "Welcome to the book" in text:
+                found_intro = True
+                break
+        self.assertTrue(found_intro)
+
+        # Cleanup
+        dialog.close()
+
+if __name__ == "__main__":
+    unittest.main()