[kokotatan]

Description

With use_pyproject: true, the GitHub Action crashes before installing Black when any
[dependency-groups] array contains an {include-group = "..."} table, which the
dependency-groups specification
allows (Black's own pyproject.toml uses them):

[dependency-groups]
dev = [{include-group = "lint"}]
lint = ["black==26.5.1"]

AttributeError: 'dict' object has no attribute 'split'

find_black_version_in_array() calls .split(";") on every entry. Because
read_version_specifier_from_pyproject() already scans every dependency group (and then
project.dependencies / optional dependencies), an included group is checked on its own,
so the fix only needs to skip non-string entries. Lookup order and version handling are
unchanged; this does not add include resolution or combine specifiers across groups
(deliberately deferred in #4606).

The crash also happens when an unrelated earlier group contains an include and the Black
pin lives in a later group or in project.dependencies.

Tests: tests/test_github_action.py runs the whole action/main.py entrypoint via
runpy with subprocess.run mocked, and checks the requirement passed to pip install
and the formatter invocation. Cases cover includes before/after a pin, a pin inside the
included group, an unrelated include before a project.dependencies pin, plain project
pins, extras + markers, and the existing "unpinned" / "missing" error paths. Four of the
eight cases fail with the AttributeError above without the fix.

AI assistance (OpenAI Codex / Claude) was used to investigate and prepare this change; the
diff was reviewed before submission.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (not a style change)
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (no documentation change needed)