wiki: Add smart redirect to first available wiki page

What does this MR do and why?

Implements a smart 302 redirect for wiki sidebar navigation that prevents 404 errors when the "home" page doesn't exist.

Problem

When clicking the "Wiki" link in the sidebar:

1. Users are always redirected to /wikis/home
2. If "home" doesn't exist but other pages do, users get a 404 error
3. This 404 can be intercepted by nginx/proxy, showing a generic error instead of GitLab's helpful wiki 404 page

Solution

The new behavior (controlled by wiki_smart_redirect feature flag):

Scenario	Response
Wiki empty	302 → /wikis/home → 200 empty state
Wiki has "home" page	302 → /wikis/home → 200 shows page
Wiki has pages but no "home"	302 → /wikis/first-page → 200 shows page

Technical changes

• Added Wiki#first_page and Wiki#first_page_slug methods to determine the appropriate target page
• Changed route from hardcoded redirect to controller index action
• Added WikiActions#index with feature flag check
• Excluded index action from load_sidebar before_action (optimization - no sidebar needed for redirect)
• Added comprehensive test coverage for model and controller behavior

The feature is controlled by the wiki_smart_redirect feature flag (gitlab_com_derisk type, disabled by default) to allow gradual rollout.

🛠️ with ❤️ at Siemens

References

• Closes #578848
• Feature flag: wiki_smart_redirect

Screenshots or screen recordings

Here are a screencast showing the old and new redirect logic for the wiki page:

• Before this MR, the user is redirected to the wiki home page without considering other pages.
• After this MR, The user is redirected to an existing page if the home wiki page does not exist.

Before	After

How to set up and validate locally

• Enable the feature flag in Rails console:

  Feature.enable(:wiki_smart_redirect)

• Create a project with wiki pages but no "home" page:

  project = Project.find_by_full_path('your/project')
  wiki = project.wiki
  wiki.create_page('other-page', 'Content', format: :markdown, message: 'Create page')

• Visit http://localhost:3000/your/project/-/wikis
• Verify it redirects to /your/project/-/wikis/other-page instead of /wikis/home
• Create a home page and verify it redirects to home:

  wiki.create_page('home', 'Home content', format: :markdown, message: 'Create home')

• Disable the feature flag and verify original behavior (always redirects to /wikis/home):

  Feature.disable(:wiki_smart_redirect)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides
• Conforms to the javascript style guides (N/A - no JS changes)
• Conforms to the database guides (N/A - no DB changes)
• Conforms to the merge request performance guidelines
• Feature flag added for gradual rollout (wiki_smart_redirect)
• Group wiki behavior verified (uses same shared WikiActions concern)