[9.3](backport #6698) Replace elastic/infra GitHub API check with public GCS branches endpoint

[mergify]

The DRA pipeline was checking branch availability by querying the private elastic/infra GitHub repo via the GitHub API, requiring a GITHUB_REPO_TOKEN. This should instead use the public endpoint https://storage.googleapis.com/artifacts-api/snapshots/branches.json.

Changes

• .buildkite/hooks/pre-command
  • Removed GITHUB_REPO_TOKEN dependency
  • Replaced check_if_file_exist_in_repo() (GitHub API + file path lookup) with check_if_branch_available() (public GCS JSON endpoint)
  • Uses jq any(. == $branch) for exact branch name matching — avoids false positives from substring matches
  • Updated call site to match new single-argument signature

# Before: queried private elastic/infra repo via GitHub API
check_if_file_exist_in_repo() {
    local path_to_file="cd/release/release-manager/project-configs/${branchName}/build.gradle"
    local response=$(curl --fail -s -H "Authorization: token $GITHUB_REPO_TOKEN" \
        https://api.github.com/repos/elastic/${repoName}/contents/${path_to_file} | ...)
}

# After: queries public GCS endpoint
check_if_branch_available() {
    local url="https://storage.googleapis.com/artifacts-api/snapshots/branches.json"
    local response=$(curl --fail -s "${url}")
    if echo "${response}" | jq -e --arg branch "${branch}" 'any(. == $branch)' > /dev/null 2>&1; then
        ...
    fi
}

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://storage.googleapis.com/artifacts-api/snapshots/branches.json
  • Triggering command: /usr/bin/curl curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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This is an automatic backport of pull request #6698 done by [Mergify](https://mergify.com).