MR title pattern merge check should ignore Draft: prefix

Summary

When a project has a required MR title pattern configured, marking an MR as a draft causes the title pattern merge check to fail — even when the underlying title (without the Draft: prefix) satisfies the pattern. This results in a confusing UX: the user sees an "Edit" call-to-action for the title check, but the only action they actually need to take is to mark the MR as ready.

Steps to reproduce

  1. Configure a required MR title pattern on a project (e.g. a regex like ^feat|fix|chore).
  2. Create an MR with a title that satisfies the pattern (e.g. feat: add new feature).
  3. Mark the MR as a draft — the title becomes Draft: feat: add new feature.
  4. Observe the merge checks widget: the title pattern check now fails, even though the underlying title is valid.

What is the current bug behavior?

The title pattern merge check evaluates the full MR title, including the Draft: prefix. If the regex does not account for the prefix, the check fails and the user is shown an "Edit" call-to-action for the title — even though the title itself is correct and the only thing blocking merge is the draft status (which is already surfaced by a separate merge check).

What is the expected correct behavior?

The title pattern merge check should strip any draft prefix (Draft: / WIP:) from the title before evaluating it against the required pattern. Since draft status already blocks merging via its own dedicated check, the title pattern check should not surface a spurious failure for draft MRs whose underlying title is valid.

Relevant logs and/or screenshots

Possible fixes

Strip the draft/WIP prefix from the MR title before running it through the title pattern regex in the mergeability check. This is analogous to the approach taken in !249733 (merged), which already handles draft prefix stripping in a related context. The mergeability checks are backend-only, so this should be a purely BE change.