Word ladder (shortest path)

You are completing a coding task in one shot. Write complete files that satisfy the spec below. Tests (pytest) will be run against your files; you cannot run them yourself.

Reply with ONE JSON object and nothing else:
{"files": {"<relative path>": "<complete file content>", ...}}

Required deliverable files: ladder.py

SPEC:
Create ladder.py with exactly one function:

ladder_length(begin: str, end: str, word_list: list[str]) -> int
    A transformation sequence goes from begin to end, changing exactly one letter per step; every intermediate word (and end itself) must appear in word_list. Return the number of words in the SHORTEST such sequence counting both endpoints, or 0 if none exists. begin does not need to be in word_list. All words are lowercase and the same length.

Example: begin="hit", end="cog", word_list=["hot","dot","dog","lot","log","cog"] -> 5 (hit -> hot -> dot -> dog -> cog).

Use only the Python standard library. A breadth-first search is the intended approach; brute-force recursion will be too slow for the larger test.

```json
{"files": {"ladder.py": "from collections import deque\n\ndef ladder_length(begin: str, end: str, word_list: list[str]) -> int:\n    if begin == end:\n        return 0\n    visited = set([begin])\n    queue = deque([(begin, 1)])\n    while queue:\n        current_word, steps = queue.popleft()\n        for i in range(len(current_word)):\n            for c in 'abcdefghijklmnopqrstuvwxyz':\n                new_word = current_word[:i] + c + current_word[i+1:]\n                if new_word == end:\n                    return steps + 2\n                if new_word in word_list and new_word not in visited:\n                    visited.add(new_word)\n                    queue.append((new_word, steps + 1))\n    return 0"}}

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=================================== FAILURES ===================================
_____________________________ test_classic_example _____________________________
test_ladder.py:7: in test_classic_example
    assert ladder_length("hit", "cog", ["hot", "dot", "dog", "lot", "log", "cog"]) == 5
E   AssertionError: assert 6 == 5
E    +  where 6 = ladder_length('hit', 'cog', ['hot', 'dot', 'dog', 'lot', 'log', 'cog'])
_____________________________ test_unreachable_end _____________________________
test_ladder.py:11: in test_unreachable_end
    assert ladder_length("hit", "cog", ["hot", "dot", "dog", "lot", "log"]) == 0
E   AssertionError: assert 6 == 0
E    +  where 6 = ladder_length('hit', 'cog', ['hot', 'dot', 'dog', 'lot', 'log'])
_______________________________ test_single_step _______________________________
test_ladder.py:15: in test_single_step
    assert ladder_length("a", "c", ["a", "b", "c"]) == 2
E   AssertionError: assert 3 == 2
E    +  where 3 = ladder_length('a', 'c', ['a', 'b', 'c'])
____________________________ test_shortest_is_found ____________________________
test_ladder.py:20: in test_shortest_is_found
    assert ladder_length("hit", "cot", words) == 3
E   AssertionError: assert 4 == 3
E    +  where 4 = ladder_length('hit', 'cot', ['hot', 'hit', 'hat', 'cat', 'cot', 'cog', ...])
__________________________ test_larger_input_is_fast ___________________________
test_ladder.py:26: in test_larger_input_is_fast
    assert ladder_length("aaa", "hhh", words) == 4
E   AssertionError: assert 5 == 4
E    +  where 5 = ladder_length('aaa', 'hhh', ['aaa', 'aab', 'aac', 'aad', 'aae', 'aaf', ...])
=========================== short test summary info ============================
FAILED test_ladder.py::test_classic_example - AssertionError: assert 6 == 5
FAILED test_ladder.py::test_unreachable_end - AssertionError: assert 6 == 0
FAILED test_ladder.py::test_single_step - AssertionError: assert 3 == 2
FAILED test_ladder.py::test_shortest_is_found - AssertionError: assert 4 == 3
FAILED test_ladder.py::test_larger_input_is_fast - AssertionError: assert 5 == 4
5 failed in 0.04s