Register the factor label for a bare labeled dict in env_list

[dylanpulver]

docs/reference/config.rst:130 says an env_list item may be "a labeled dict ({ ecosystem = ["oci", "python"] }) that also registers a {factor:label} name". It does not.

env_list = [{ ecosystem = ["oci", "python"] }]
[env_run_base]
description = "Sync {factor:ecosystem} artifacts"

tox c -e oci -k description → Sync artifacts. The identical group inside a product dict → Sync oci artifacts.

Cause

_extract_env_list_labels only walks items with a "product" key — correct for #3865, added when product was the only dict shape env_list took. #3923 then taught TomlLoader.to_env_list three shapes and updated these docs, but never touched the source one directory over. The sibling path for [env_base.*].factors already registers bare groups.

Fix

Register the label for a bare labeled dict too, honouring default as the product branch does. extract_label returns None for a range dict, so bare ranges are unaffected.

Verification

Reverting the source fails both new tests; a naive variant that registers the label but drops default fails the second. tests/config: 6687 passed, 2 skipped. ty, mypy, pyrefly and pre-commit clean.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation (the docs already describe this)