Add test coverage for to_pickle export

[GiGiKoneti]

Describe your changes

to_pickle is a public export function in save.py but had zero test
coverage — unlike to_pyg which is already covered by test_save_to_pyg.

Adds test_save_to_pickle to tests/test_save.py mirroring the same
structure. The test does a full round-trip check:

• output .pkl file is created on disk at the expected path
• file loads back cleanly with pickle.load()
• loaded object is a networkx graph instance
• node and edge counts match the original after deserialisation

No production code touched. No new dependancies introduced.

Issue Link

Related to #88

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

Checklist before requesting a review

• My branch is up-to-date with the target branch - if not update your fork with the changes from the target branch (use pull with --rebase option if possible).
• I have performed a self-review of my code
• For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
• I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
• I have updated the documentation to cover introduced code changes
• I have added tests that prove my fix is effective or that my feature works
• I have given the PR a name that clearly describes the change, written in imperative form (context).
• I have requested a reviewer and an assignee (assignee is responsible for merging)

Checklist for reviewers

Each PR comes with its own improvements and flaws. The reviewer should check the following:

• the code is readable
• the code is well tested
• the code is documented (including return types and parameters)
• the code is easy to maintain

Author checklist after completed review

• I have added a line to the CHANGELOG describing this change, in a section
  reflecting type of change (add section where missing):
  • added: when you have added new functionality
  • changed: when default behaviour of the code has been changed
  • fixes: when your contribution fixes a bug

Checklist for assignee

• PR is up to date with the base branch
• the tests pass
• author has added an entry to the changelog (and designated the change as added, changed or fixed)
• Once the PR is ready to be merged, squash commits and merge the PR.

[leifdenby]

Thank you for adding this, but aren't you just checking that pickle works with this test? I don't quite understand what this adds

[GiGiKoneti]

Hey @leifdenby

I know it looks like it's just testing the standard library pickle, but the main reson is to guarantee that our specific, complex graph objects (like the ones from create_oskarsson_hierarchical_graph) are completely serializable. Sometimes custom networkx attributes can act wierd when pickled, so this ensures a safe round-trip.It also explicitly tests the public I/O contract in save.py. If we ever decide to refactor to_pickle later (like adding gzip compression or stripping unnessecary nodes to save disk space before export), we'll have a regression test already backing it up. Mostly just wanted to bring the coverage on par with to_pyg since thats tested too. Let me know if you want me to add checks for any deeper graph properties!

[bhawna346]

Hi, I’d like to work on this issue. Could you please assign it to me?

[GiGiKoneti]

This is a PR.

[bhawna346]

Thanks for the clarification! I'll look for open issues instead.