## 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
8d0cdfc93e/src/pydocstyle/checker.py (L581C13-L581C64)
## Example
Here is a playground example -
https://play.ruff.rs/eab9ea59-92cf-4e44-b1a9-b54b7f69b178
```py
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:
```bash
$ 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.
d3f51cf3a6