Fix extract_columns fill_with for columns passed as (name, doc) tuples

[breken-ai]

@extract_columns accepts columns as plain names or as (name, doc) tuples. With fill_with= set, the filler looped over the raw arguments, so documented columns were never filled. It checked ("b", "doc b") in df, added a junk column named after the whole tuple, and left b missing. Requesting b then failed.

@extract_columns(("a", "doc a"), ("b", "doc b"), fill_with=0)
def df() -> pd.DataFrame:
    return pd.DataFrame({"a": [1, 2]})

dr.execute(["b"])
# InvalidDecoratorException: No such column: b produced by df.
# It only produced Index(['a', ('a', 'doc a'), ('b', 'doc b')], dtype='object')

The same call with plain names (extract_columns("a", "b", fill_with=0)) works.

Changes

• hamilton/function_modifiers/expanders.py: extract_columns.transform_node computes the column names once (unwrapping (name, doc) tuples, as the extractor loop below already does). The sync and async df_generators use those names to fill missing columns.
• tests/function_modifiers/test_expanders.py: two regressions (sync and async) for fill_with with documented columns.

How I tested this

• New tests on unmodified main (661aa888): both fail. Sync: the dataframe columns are ['col_1', ('col_1', 'col_1 doc'), ('col_3', 'col_3 doc')] instead of ['col_1', 'col_3']. Async fails the same way.
• With the fix, both pass.
• pytest tests/function_modifiers tests/test_function_modifiers.py tests/test_end_to_end.py tests/test_hamilton_driver.py tests/test_async_driver.py: 529 passed.
• ruff check and ruff format --check (0.15.14, the pre-commit version) are clean on the changed files. git diff --check is clean.

Notes

The filler has iterated over self.columns like this since at least 0857e5d (2023-03).

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output) — no new functions
• Placeholder code is flagged / future TODOs are captured in comments — none added
• Project documentation has been updated if adding/changing functionality — no documented behavior changes; this makes the code match the existing docs

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AI disclosure (per the ASF Generative Tooling guidance): this change was written with an AI coding tool (Claude Code, Claude Opus 5.5) working through the breken-ai account. The tool found the bug, wrote the fix and the test, and wrote this description. The commit carries a Generated-by: trailer. The diff is a small original change to existing Hamilton code and includes no third-party material. The red/green runs above are real and can be re-run from the diff. If you would rather not take AI-assisted contributions here, say so and I will close this.