Fix O(n) memory growth in PDF conversion by calling page.close() afte… (#1612)

* Fix O(n) memory growth in PDF conversion by calling page.close() after each page

* Refactor PDF memory optimization tests for improved readability and consistency

* Add memory benchmarking tests for PDF conversion with page.close() fix

* Remove unnecessary blank lines in PDF memory optimization tests for cleaner code

* Bump version to 0.1.6b2 in __about__.py

* Update PDF conversion tests to include mimetype in StreamInfo