Fix docstring indentation with leading tabs

[puneetdixit200]

Description

Closes #3667.

Black expands leading tabs in multiline docstrings before it computes the docstring indentation to strip. That expansion used Python's default 8-column tab stops, while Black normalizes surrounding code indentation to 4 spaces. As a result, nested tabs inside indented docstrings were doubled in the formatted output.

This changes the docstring-only leading-tab expansion to use 4-space columns, updates the existing tabbed docstring fixture, and adds a focused regression case from the issue shape.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

Tests

• .venv/bin/python -m pytest tests/test_format.py -k 'docstring or multiline_strings or string_prefixes' -q
• .venv/bin/python -m pytest tests/test_black.py -k 'docstring or lines_with_leading_tabs_expanded' -q
• .venv/bin/python -m pytest tests --run-optional no_jupyter -n auto -q -k 'not expression_diff_with_color and not piping_diff_with_color'
• env -u NO_COLOR TERM=xterm-256color .venv/bin/python -m pytest tests/test_black.py -k 'expression_diff_with_color or piping_diff_with_color' -q
• .venv/bin/python -m black --check --verbose src/black/strings.py tests/test_black.py
• SKIP=prettier PRE_COMMIT_HOME="$PWD/.pre-commit-cache" PIP_CACHE_DIR="$PWD/.pip-cache" .venv/bin/pre-commit run --files CHANGES.md src/black/strings.py tests/test_black.py tests/data/cases/docstring.py tests/data/cases/docstring_tabs.py

The prettier hook was skipped locally because nodeenv failed SSL certificate verification while downloading Node; the other pre-commit hooks passed.

[github-actions]

diff-shades results comparing this PR (988ffff) to main (13e97b4):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[cobaltt7]

Thanks!