test: improve coverage for notebook manipulator and converter

[allenporter]

Summary

• Added unit tests in tests/notebook/test_manipulator.py covering NotebookBuilder, notebook reconstruction (reconstruct), notebook merging (merge), page addition/deletion/reordering, layer modifications, stroke path manipulation, and keyword/title/link packing.
• Expanded unit tests in tests/notebook/test_converter.py covering SvgConverter (vector stroke tracing, visibility overlay, custom palettes), PdfConverter (single/multi-page export, keyword bookmarks, page links, web links, landscape pages), and TextConverter edge cases.
• Increased test coverage for supernote/notebook/manipulator.py from 10% to 99% and supernote/notebook/converter.py from 64% to 93%.
• Verified all 396 tests pass via ./script/test.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.05%. Comparing base (cc75a73) to head (898978c).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   79.30%   83.05%   +3.75%     
==========================================
  Files         103      103              
  Lines       10584    10584              
==========================================
+ Hits         8394     8791     +397     
+ Misses       2190     1793     -397     

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