BUG: Fix PageRange.__add__ crashing with TypeError when start or stop is None

[gaoflow]

Problem

PageRange.__add__ raises TypeError: '>' not supported between instances of 'int' and 'NoneType' (or NoneType and int, or NoneType and NoneType) whenever either operand has a None start or None stop, which is perfectly normal for ranges like "5:" or ":3".

>>> from pypdf.pagerange import PageRange
>>> PageRange("0:5") + PageRange("3:")   # stop is None
TypeError: '>' not supported between instances of 'NoneType' and 'int'

>>> PageRange(":5") + PageRange(":3")    # both starts are None
TypeError: '>' not supported between instances of 'NoneType' and 'NoneType'

The problem is that __add__ compares slice.start and slice.stop with > directly. None bounds are not comparable to integers, but they are semantically well-defined: None start = "beginning of sequence" and None stop = "end of sequence".

Fix

Add two small helper functions inside __add__ that map None to float sentinels before any comparison:

• _start_key(None) → -inf (beginning sorts before any page index)
• _stop_key(None) → +inf (end sorts after any page index)

Replace the direct comparisons and max() call with these key functions. No public API changes.

Behaviour after fix

>>> PageRange("0:5") + PageRange("3:")   # => PageRange("0:")
>>> PageRange(":5")  + PageRange(":3")   # => PageRange(":5")
>>> PageRange("5:")  + PageRange(":3")   # => ValueError: gap (correct)
>>> PageRange(":")   + PageRange("2:7")  # => PageRange(":")

All 23 existing tests continue to pass.

---

This pull request was prepared with the assistance of AI, under my direction and review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.86%. Comparing base (75e339c) to head (3a1e152).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3898      +/-   ##
==========================================
+ Coverage   97.85%   97.86%   +0.01%     
==========================================
  Files          57       57              
  Lines       10702    10743      +41     
  Branches     2001     2006       +5     
==========================================
+ Hits        10472    10514      +42     
  Misses        127      127              
+ Partials      103      102       -1     

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[stefan6419846]

Please avoid unnecessary changes.

[gaoflow]

Restored in 3a1e152a. Verification passed:

python -m pytest tests/test_pagerange.py -q
python -m ruff check pypdf/pagerange.py tests/test_pagerange.py

[stefan6419846]

Please avoid unnecessary changes.

[gaoflow]

Restored in 3a1e152a. Verification passed:

python -m pytest tests/test_pagerange.py -q
python -m ruff check pypdf/pagerange.py tests/test_pagerange.py

[gaoflow]

Reverted both unrelated reformatting changes (the parse_filename_page_ranges signature and the raise ValueError line) in 3a1e152 — the diff is now limited to the __add__ None-bounds fix. Ready for another look.