Preserve function docstring spacing with line ranges

[happykawayigt]

Description

Fixes #5285.

When line-range formatting replaces out-of-range code with fmt: pass placeholders, those placeholders are represented as comment lines. The blank-line tracker could therefore scan past the enclosing function boundary and misclassify its docstring as a module docstring, adding a blank line that was outside the requested formatting range.

Treating a converted fmt: pass line as a scope boundary preserves the existing spacing. The regression fixture exercises every single-line range across a formatted function whose docstring has no following blank line.

Validation:

• pytest tests/test_format.py -q (230 passed)
• pre-commit on changed files: isort, flake8, mypy, prettier, EOF, trailing whitespace passed
• git diff --check

AI assistance was used during implementation. I independently reviewed the line-range conversion path and final diff, reproduced the regression through the line-by-line fixture, and ran the validation above.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (No style change.)
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (No documentation change is needed.)

[github-actions]

diff-shades results comparing this PR (8542127) to main (5706d79):

--preview style: no changes

--stable style: no changes

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

[cobaltt7]

Is there a reason this is a draft PR? It all looks good to me!