Draft: Let the title pattern choose the auto-remediation title format

What does this MR do and why?

Closes #634938.

A project can set a merge request title pattern, and an MR whose title fails it cannot merge. Auto-remediation chose plain or conventional titles from recent history only, so a project that just adopted a conventional pattern, with mostly plain history, got Security: Update ... MRs nobody could merge without renaming them. Raised in review of the Conventional Commits change.

• History still picks the style. When the project's pattern rejects the resulting title, the other format is used if it passes.
• The pattern only validates, so the conventional fallback uses build(deps): [security] update ....
• When neither format passes, the history choice stays and the mismatch is logged.
• The commit subject follows the same choice, since in a conventional repository the subject is the title.
• Behind the same dependency_management_conventional_commits flag as the conventional format. With the flag off, the pattern is not consulted and titles stay plain.
• Matching uses Gitlab::UntrustedRegexp.with_fallback, as the merge check does, and only when the merge_request_title_regex_check licensed feature is available.

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