diff: filter ignored nested keys from object values

[gaoflow]

Summary

• reuse the existing ignore path check when copying values into diff payloads
• prune ignored dictionary subkeys from whole-object change/add/remove values
• add regression coverage for the Getting unexpected value in diff when using ignore in add/remove object case #187 change case and the matching add/remove object cases

Closes #187.

Validation

• python -m pytest tests/test_dictdiffer.py::DictDifferTests::test_ignore_nested_key_in_object_values -q -o addopts=''
• python -m pytest -q -o addopts=''
• python -m compileall dictdiffer tests
• git diff --check
• python -m pycodestyle --ignore=E501,W504 dictdiffer/__init__.py tests/test_dictdiffer.py

Note: the repository default pytest addopts require optional pytest lint plugins that are not installed in this local environment, so the test commands above clear addopts for behavioral validation.

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