Add Gitlab::Git::Finders::BranchesFinder

What does this MR do and why?

Contributes to #419412 (closed)

Problem

The existing app-level BranchesFinder uses the FindLocalBranches Gitaly RPC which lacks server-side search support, requiring all branches to be fetched and filtered in Ruby.

Solution

Create a new Gitlab::Git::Finders::BranchesFinder that uses the ListRefs Gitaly RPC via RefsFinder for server-side search and pagination. Features include:

• Search operators: ^ (starts with), $ (ends with), * (wildcard)
• Sorting: name_asc, name_desc, updated_asc, updated_desc
• Cursor-based pagination via per_page and page_token
• Optional commit hydration via include_commits parameter

References

• Issue: #419412 (closed)
• Epic: &11057

Screenshots or screen recordings

Not applicable - backend changes only.

How to set up and validate locally

1. Open a Rails console: bin/rails console
2. Test the new finder:

project = Project.find_by_full_path('gitlab-org/gitlab')
repo = project.repository.raw_repository

# Basic search
finder = Gitlab::Git::Finders::BranchesFinder.new(repo, { search: 'master' })
finder.execute

# With pagination
finder = Gitlab::Git::Finders::BranchesFinder.new(repo, { per_page: 5 })
branches = finder.execute
finder.next_cursor  # cursor for next page

# With commit hydration
finder = Gitlab::Git::Finders::BranchesFinder.new(repo, { search: 'main' }, include_commits: true)
branches = finder.execute
branches.first.dereferenced_target  # commit object present

MR acceptance checklist

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