Preview: Consider erroring if direct code is used when mode is disabled

In https://github.com/astral-sh/ruff/pull/7210/commits/167aeeefe23e18c251cf67065cc6e5d55c24f060 we demonstrate that a warning is displayed when a preview rule is selected with a direct (exact) code but preview mode is disabled. We may want make this an error instead (and exit with a non-zero code).

Start by reviewing the linked Ruff pull request and commit to understand how the warning is currently produced when a preview rule is selected by exact code with preview mode disabled. Trace that behavior to the relevant CLI entry point, then verify that the condition becomes an error and exits with a non-zero status.

Written by the indexing model from the issue text.