Fix preview split for commented method chains

[ishaanlabs-gg]

Description

Fixes #3998.

When a standalone comment appears between chained method calls, Black could split the
same leading call differently depending on whether another chained call followed the
comment. This adds a preview-gated formatter feature so commented method chains split
the first trailer consistently.

The change is intentionally limited to dotted delimiter splitting when the line contains
a standalone comment. Stable style remains unchanged.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

Validation:

• PYTHONPYCACHEPREFIX=.pycache-tmp /Volumes/STOREJET/2/Dependencies/black-3998-venv/bin/python -m pytest tests/test_format.py -k preview_method_chain_comment -q
• PYTHONPYCACHEPREFIX=.pycache-tmp /Volumes/STOREJET/2/Dependencies/black-3998-venv/bin/python -m pytest tests/test_docs.py tests/test_schema.py -q
• PYTHONPYCACHEPREFIX=.pycache-tmp /Volumes/STOREJET/2/Dependencies/black-3998-venv/bin/python -m pytest tests/test_format.py -q
• git diff --check

[cobaltt7]

This is a design decision, so we need to discuss the pros and cons of both styles.

[github-actions]

diff-shades results comparing this PR (b747e2d) to main (74371e2):

--preview style (View full diff):

╭─────────────────────── Summary ────────────────────────╮
│ 4 projects & 11 files changed / 48 changes [+32/-16]   │
│                                                        │
│ ... out of 3 037 312 lines, 14 117 files & 22 projects │
╰────────────────────────────────────────────────────────╯
Differences found.

--stable style: no changes

---

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

[cobaltt7]

Sorry, I misinterpreted the intended scope of this PR. After reviewing the original issue, it's clear this PR is meant to fix an issue I would personally consider a bug. However, the current implementation seems to be a bit overzealous. Here's some changes from the diff-shades report that seem detrimental, and the formatting should remain unchanged:

My understanding of the original issue is that fluent calls should never be split if they're a single call or if they fit inside the line length. If neither of those are true, then all calls in the expression should be split. The issue is that comments between multiple calls that should split can prevent the calls from splitting. It looks like this PR also allows comments before the entire fluent expression, or comments nested inside calls to cause the expression to be split.