[pydocstyle] Split on first whitespace character (D403)

[my1e5]

Summary

This PR fixes an issue where Ruff's D403 rule (first-word-uncapitalized) was not detecting some single-word edge cases that are picked up by pydocstyle.

The change involves extracting the first word of the docstring by identifying the first whitespace character. This is consistent with pydocstyle which uses .split() - see https://github.com/PyCQA/pydocstyle/blob/8d0cdfc93e86e096bb5753f07dc5c7c373e63837/src/pydocstyle/checker.py#L581C13-L581C64

Example

Here is a playground example - https://play.ruff.rs/eab9ea59-92cf-4e44-b1a9-b54b7f69b178

def example1():
    """foo"""

def example2():
    """foo
    
    Hello world!
    """

def example3():
    """foo bar

    Hello world!
    """

def example4():
    """
    foo
    """

def example5():
    """
    foo bar
    """

pydocstyle detects all five cases:

$ pydocstyle test.py --select D403
dev/test.py:2 in public function `example1`:
        D403: First word of the first line should be properly capitalized ('Foo', not 'foo')
dev/test.py:5 in public function `example2`:
        D403: First word of the first line should be properly capitalized ('Foo', not 'foo')
dev/test.py:11 in public function `example3`:
        D403: First word of the first line should be properly capitalized ('Foo', not 'foo')
dev/test.py:17 in public function `example4`:
        D403: First word of the first line should be properly capitalized ('Foo', not 'foo')
dev/test.py:22 in public function `example5`:
        D403: First word of the first line should be properly capitalized ('Foo', not 'foo')

Ruff (0.8.4) fails to catch example2 and example4.

Test Plan

• Added two new test cases to cover the previously missed single-word docstring cases.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[my1e5]

Closes #13167

[dylwil3]

Good catch! This looks good to me, thank you for your contribution!

[dylwil3]

Wonderful, thanks so much for your contribution!