Added doctests to eulerian_path_and_circuit_for_undirected_graph.py

[Sinclo]

Describe your change:

Added doctests to eulerian_path_and_circuit_for_undirected_graph.py Contributes to https://github.com/TheAlgorithms/Python/issues/9943

• [ ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [x] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [x] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[Sinclo]

Hi @cclauss , @tianyizheng02

Sorry to tag as I'm sure you guys are both quite busy. But I wanted to know if it would be possible to kindly review and approve this PR if all looks good (I noticed several other newer PRs have already been reviewed and merged; this one has been open since October 2024). Would love to contribute to your project.

Thank you for your time!