Add UI to configure merge request title validation regex in project settings

Summary

Add user interface to Project Settings > Merge Requests to configure the merge request title validation regex. The backend infrastructure already exists (GraphQL fields: mergeRequestTitleRegex, mergeRequestTitleRegexDescription), but there's no UI to configure it.

Problem to Solve

Customers cannot access the MR title validation feature because it's only available via API/GraphQL. This blocks adoption of a highly-requested feature (133+ upvotes on #25689).

Proposal

Add a new section to Settings > Merge requests with:

1. Merge request title validation section
2. Text input for regex pattern (e.g., ^(feat|fix|docs|chore): .+)
3. Text area for description/help text shown to users when validation fails
4. Checkbox to enable/disable validation
5. Preview/test functionality (optional for v1)

UI Mockup Needed

• UX review for placement and layout
• Error state design when regex is invalid
• Help text explaining regex syntax

Implementation Notes

GraphQL Fields (already exist):

• mergeRequestTitleRegex (String)
• mergeRequestTitleRegexDescription (String)

API Endpoints:

• Use existing project update mutation
• Validate regex pattern on save

Validation:

• Ensure regex is valid before saving
• Show preview of what titles would/wouldn't match (nice-to-have)

Acceptance Criteria

• Settings UI appears in Project Settings > Merge Requests
• Maintainers can set a regex pattern for MR titles
• Maintainers can set a description explaining the pattern
• Invalid regex patterns show clear error messages
• Settings persist correctly via GraphQL mutation
• UI shows current values when editing
• Help text explains the feature and provides examples

Related Issues

• Parent: gitlab-org#20108 (Epic)
• Related: #25689 (original feature request)
• Related: #16080 (customize default MR title)

Documentation

• Update project settings documentation
• Add examples for common patterns (Jira, conventional commits)