chore: remove obsolete FIXME comments from bipartite tests

[dwaddle]

Describe your change:

Removed 4 obsolete FIXME comments from graphs/check_bipatrite.py that were left behind in a previous PR (#10708). The bugs they referenced (KeyError and TypeError) were already fixed at that time by migrating to a defaultdict and adding node existence checks, making these FIXME comments and tracebacks redundant. No functional code or tests were changed.

• 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".

[cclauss]

@priya-sundaram-dev, your review. please. Compare with #15210 (review)

[priya-sundaram-dev]

Reviewed, @cclauss — this one is a clean approve from me, and it's a nice contrast with #15210.

Both PRs target the same FIXME comments in check_bipatrite.py, but they take opposite approaches:

• #15210 deleted the guards (if node not in graph: ...) that the FIXMEs sat next to. As I noted there, those guards are load-bearing — they make an ordinary adjacency dict where a node appears as a neighbor but is omitted as a key (a sink with no out-edges) work correctly. Removing them breaks 13 existing doctests and CI goes red.
• #15107 deletes only the comments, leaving the working code untouched.

That's the right call, because the FIXMEs were simply wrong: they claimed the adjacent doctests "should fail with KeyError / TypeError", but those inputs are handled gracefully and the doctests already assert the correct False/True results. So the comments were stale/misleading, not a real TODO. Removing them is documentation cleanup with zero behavior change.

Verified locally on this branch: pure deletions (no code touched), and python -m doctest graphs/check_bipatrite.py → 30 passed, 0 failed. CI here is green across build / build_docs / ruff / pre-commit.

LGTM. 👍 If we want to also honor the literal ask in #15127 (let invalid graphs raise), that's a separate design decision — it would need the doctests rewritten to # doctest: +IGNORE_EXCEPTION_DETAIL style expected-exception blocks — and shouldn't block this cleanup.