Add write option for Repositories in job token fine-grained permissions

What does this MR do and why?

During staging verification of the allow_push_to_allowlisted_projects feature flag, it was found that the allowlist's per-resource fine-grained form has no admin/write option for Repositories — only None/Read. Every other resource with a backend admin policy renders "Read and write" in the same form.

app/assets/javascripts/token_access/constants.js (POLICIES_BY_RESOURCE) was missing a POLICY_ADMIN_REPOSITORIES entry, although the backend (lib/ci/job_token/policies.rb) already accepts :admin_repositories.

This MR is a frontend-only fix: it adds POLICY_ADMIN_REPOSITORIES to app/assets/javascripts/token_access/constants.js (plus the matching test mock), which surfaces the missing Read and write option in the Repositories dropdown. No backend changes are needed.

Validation:

• With fine-grained ON and Repositories set to "Read and write", cross-project push succeeds.
• With Repositories set to "Read" only, push correctly returns 403 — confirming both branches of policies_allowed?(..., [:admin_repositories]) work as expected.

References

• Feature flag rollout issue: #597258 (closed)
• Feature issue: #479907 (closed)
• Original implementation MR: !229671 (merged) (backend)
• Original implementation MR: !232351 (merged) (frontend)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the allow_push_to_allowlisted_projects feature flag in GDK.
2. Navigate to a target project's Settings > CI/CD > Token Access.
3. Enable inbound job token scope and add a source project to the allowlist.
4. Enable fine-grained permissions for the allowlist entry.
5. Verify the Repositories dropdown now shows "None", "Read", and "Read and write".
6. Set Repositories to "Read and write" and confirm a cross-project git push from the source project's CI pipeline succeeds.
7. Set Repositories back to "Read" and confirm the push returns 403.

MR acceptance checklist

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