Fix list of keys before modifying that same dictionary

[rolandwalker]

Description

Fixes a bug in #1653.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Missing regression test for the exact bug fixed (medium)
   The PR changes iteration to for warning_token in list(style.keys()) in mycli/clistyle.py:189, which prevents mutating a dict during iteration. There is no corresponding test to lock this behavior.
   Action: add a unit test for style_factory_helpers(..., warnings=True) that asserts it does not raise and that Token.Output.* entries are created from Token.Warnings.*.

2. Current style tests don’t protect this path (medium)
   Existing tests in test/test_clistyle.py are skipped, so this fix is currently unguarded by active tests.
   Action: add/enable a non-skipped test for style_factory_helpers (not only style_factory_toolkit) covering warnings-style token copying.

Security concerns found in this PR: none.

I couldn’t execute tests in this environment (pytest and required deps are unavailable), so runtime verification here is limited.