fix: keep docstring-only Python chunks valid

[Elonon901001]

Summary

Fixes #12330.

When strip_docstrings=True removes the only statement in a function or class, keep the generated chunk valid by inserting an indented pass. Add a regression test that parses docstring-only functions and classes with ast.parse, plus the required release note.

Validation

• hatch run test:unit --no-cov test/components/preprocessors/test_python_code_splitter.py -q — 71 passed.
• git diff --check

I used an AI assistant to help prepare this PR, reviewed the changes, and ran the relevant tests.

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Hi @Elonon901001, thanks for your interest in contributing to Haystack! 🙏

⚠️ Issue #12330 is already being addressed by open pull request(s) #12331. Before opening a PR for an issue, please check whether a PR is already linked to it, and consider contributing to the existing PR instead. We may close duplicate PRs to keep the review queue manageable.

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