Support full refs in default_branch for Projects API

What does this MR do and why?

Contributes to #516857

Problem

The Projects API doesn’t support full references for the default_branch setting when creating a project via the Projects API. This leads to a non-initialized state.

Solution

Allow the default_branch parameter to accept fully qualified references (e.g., refs/heads/main) and automatically convert them to branch names (e.g., main).

References

API: Creating a Project with a full ref as Defa... (#516857)

How to set up and validate locally

1. Provide token and create a project via REST API

curl --request POST --header "PRIVATE-TOKEN: <your-token>" \
     --header "Content-Type: application/json" --data '{
        "name": "default_branch_project", "description": "New Project", "path": "default_branch_project", "default_branch": "refs/heads/custom_main",
        "namespace_id": "1", "initialize_with_readme": "true"}' \
     --url "http://gdk.test:3000/api/v4/projects/"

2. Visit new project.
3. You should see a default branch named custom_main

MR acceptance checklist

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Related to #516857