Add previousBlamePath to blame CommitData GraphQL type

What does this MR do and why?

Adds a previousBlamePath field to the CommitData GraphQL type (blame data). This MR exposes a Rails-built path (project_blob_path(project, tree_join(parent_id, previous_path), blame: 1)) through Gitlab::BlamePresenter, so the frontend can consume a root-correct URL instead of assembling its own.

A follow-up frontend MR (stacked on this one) switches the inline blame component to consume this field and removes the client-side URL assembly.

References

• Resolves (together with the frontend MR): #627734 (closed)
• Frontend guideline this follows: https://docs.gitlab.com/development/urls_in_gitlab/ (generate URLs in Rails, pass to frontend)
• Same class of bug previously fixed in !245993 (merged)

Screenshots or screen recordings

Not applicable — GraphQL API change only, no UI change in this MR.

How to set up and validate locally

1. Open GraphiQL at http://gdk.test:3000/-/graphql-explorer

2. Run:

   {
     project(fullPath: "flightjs/Flight") {
       repository {
         blobs(ref: "master", paths: ["README.md"]) {
           nodes {
             blame(fromLine: 1, toLine: 5) {
               groups {
                 previousPath
                 commitData {
                   previousBlamePath
                 }
               }
             }
           }
         }
       }
     }
   }

3. previousBlamePath should be /<group>/<project>/-/blob/<parent-sha>/<path>?blame=1, and null for groups whose commit has no parent.

4. With relative_url_root: /gitlab set in gdk.yml, the path should include the /gitlab prefix.

MR acceptance checklist

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