[`pydoclint`] Fix `SyntaxError` from fixes with line continuations (`D201`, `D202`) (#19246)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This PR fixes #7172 by suppressing the fixes for
[docstring-missing-returns
(DOC201)](https://docs.astral.sh/ruff/rules/docstring-missing-returns/#docstring-missing-returns-doc201)
/ [docstring-extraneous-returns
(DOC202)](https://docs.astral.sh/ruff/rules/docstring-extraneous-returns/#docstring-extraneous-returns-doc202)
if there is a surrounding line continuation character `\` that would
make the fix cause a syntax error.
To do this, the lints are changed from `AlwaysFixableViolation` to
`Violation` with `FixAvailability::Sometimes`.
In the case of `DOC201`, the fix is not given if the non-break line ends
in a line continuation character `\`. Note that lines are iterated in
reverse from the docstring to the function definition.
In the case of `DOC202`, the fix is not given if the docstring ends with
a line continuation character `\`.
## Test Plan
<!-- How was it tested? -->
Added a test case.
ruff
966fd6f5 - [`pydoclint`] Fix `SyntaxError` from fixes with line continuations (`D201`, `D202`) (#19246)
