Add to doctests - #12790

Describe your change:

Contributes to #9943
Added doctests for the following functions in the Graph class inside directed_and_undirected_weighted_graph.py
add_pair
remove_pair
dfs
bfs
has_cycle

Confirmed all doctests pass with python -m doctest -v

Note:
has_cycle only checks for cycles in the connected components of the starting node. It does not detect cycles in disconnected or self-loops. These limitations have been documented in the function's docstring.

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

Checklist:

• [ * ] I have read CONTRIBUTING.md.
• [ * ] This pull request is all my own work -- I have not plagiarized.
• [ * ] I know that pull requests will not be merged if they fail the automated tests.
• [ * ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ * ] All new Python files are placed inside an existing directory.
• [ * ] All filenames are in all lowercase characters with no spaces or dashes.
• [ * ] All functions and variable names follow Python naming conventions.
• [ * ] All function parameters and return values are annotated with Python type hints.
• [ * ] 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".