Fix 500 when creating a branch with an integer name

What does this MR do and why?

Problem

A JSON request with an integer branch name or ref reaches the sanitizer as an Integer and raises NoMethodError, returning 500.

Solution

• Convert branch names and refs to strings before sanitizing.
• Cover integer JSON params in controller specs.

References

No related issue exists. This change follows a Sentry error in rails/html/sanitizer.rb, triggered by Projects::BranchesController#create.

Screenshots or screen recordings

Not applicable. This change has no visual impact.

How to set up and validate locally

1. Run bin/rspec spec/controllers/projects/branches_controller_spec.rb.
2. Confirm the integer branch_name case returns 200 with the name "551", and the integer ref case creates the branch from an existing 551 ref (200).
3. Confirm a missing branch name follows normal validation instead of returning 500.

MR acceptance checklist

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