refactor(tests): use omniread public API instead of internal module imports

- Replace deep imports with top-level omniread exports in tests
- Ensure tests validate only the supported public API surface
- Align HTML and PDF tests with documented library usage

tests/test_pdf_simple.py

@@ -1,9 +1,13 @@
 from typing import Literal
 from pydantic import BaseModel
 
-from omniread.pdf import PDFParser
-from omniread.core.content import Content
+from omniread import (
+    # core
+    Content,
 
+    # pdf
+    PDFParser,
+)
 
 class ParsedPDF(BaseModel):
     size_bytes: int