feat: Add .values() accessor to Object (plus refactor items/values logic)

[qooxzuub]

Implements .values() for pikepdf.Object, allowing standard dictionary-style iteration over values.

• Refactored the existing .items() logic into a shared helper pydict_from_object to eliminate code duplication.
• Ensures .values() returns a standard Python dict_values view.
• Supports both Dictionaries and Streams (just like .items() and .keys()).

Added tests for standard usage, invalid UTF-8 key handling, and type errors.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.62%. Comparing base (affe28c) to head (f91bffa).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/object.cpp	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
- Coverage   93.63%   93.62%   -0.01%     
==========================================
  Files          60       60              
  Lines        6914     6920       +6     
  Branches      391      391              
==========================================
+ Hits         6474     6479       +5     
- Misses        440      441       +1     

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