Use None sentinel in Boruvka MST and un-ignore ty not-iterable rule

[iosayin]

Describe your change

Part of #15187: un-ignores the ty not-iterable rule.

Graph.boruvka_mst in graphs/minimum_spanning_tree_boruvka.py used -1 in cheap_edge to mean "no cheapest edge yet", while the real values are [head, tail, weight] lists. Because an int and a list share the same dict, the later head, tail, weight = head_tail_weight produced the only not-iterable diagnostic left in the repo:

graphs/minimum_spanning_tree_boruvka.py:190:42 error[not-iterable] Object of type `int & ~Literal[-1]` is not iterable

Changes:

• cheap_edge is now dict[int, list[int] | None], built with dict.fromkeys(...). None is the sentinel and checks use is None / is not None.
• The two copy-pasted "update cheapest edge for this component" blocks are now one small loop over (set1, set2) that reads the value into a local, so ty can narrow it without a not-subscriptable error.
• Removed rules.not-iterable = "ignore" from [tool.ty] in pyproject.toml.

Behaviour is unchanged. The existing doctest passes as before. I also compared the MST total weight and edge count against a Kruskal reference on 3,000 random connected graphs, with and without distinct_weight(), before and after the change, and found no differences.

Verification (Python 3.15):

• python -m doctest -v graphs/minimum_spanning_tree_boruvka.py: passed
• ruff check / ruff format --check on the file: clean
• ty check --exclude-scripts with the rule enabled: 0 not-iterable diagnostics, same total count as on master with the rule ignored

• 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.
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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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