[formatter] do not emit warnings that the formatter conflicts with rule missing-trailing-comma if no conflicts

[XuehaiPan]

Summary

Only emit a warning if the formatter's setting skip-magic-trailing-comma is conflicting with the linter rule missing-trailing-comma (COM812).

The following settings can co-exist with each other.

[lint]
select = [
    "COM",  # flake8-commas
]

[formatter]
skip-magic-trailing-comma = false  # the default value

Test Plan

CI

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.