[Backport 2.19] Onboard new backport-pr re-usable github workflow (opensearch-dashboards-functional-test)

[peterzhuamazon]

Description

Backport of #2074 to 2.19 branch.

Onboard new backport-pr re-usable github workflow (opensearch-dashboards-functional-test)

Issues Resolved

opensearch-project/opensearch-build#6270

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[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit 0d70185.

Path	Line	Severity	Description
.github/workflows/backport.yml	12	high	Reusable workflow referenced at mutable '@main' tag instead of a pinned commit hash: 'opensearch-project/opensearch-build/.github/workflows/backport-pr.yml@main'. Any future commit to that branch automatically changes the code executed here, creating a supply chain attack vector. The previous actions in this file were pinned to specific commit SHAs — this regression removes that protection.
.github/workflows/backport.yml	14	high	OPENSEARCH_CI_BOT_TOKEN secret is passed to an externally-controlled reusable workflow pinned at a mutable '@main' reference. If the upstream 'opensearch-build' repository is ever compromised or the 'main' branch modified, the called workflow gains full access to this privileged token and could exfiltrate it. The 'pull_request_target' trigger (which runs with repository secrets even for fork PRs) amplifies this risk.

The table above displays the top 10 most important findings.

Total: 2 | Critical: 0 | High: 2 | Medium: 0 | Low: 0

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Pull Requests Author(s): Please update your Pull Request according to the report above.

Repository Maintainer(s): You can bypass diff analyzer by adding label skip-diff-analyzer after reviewing the changes carefully, then re-run failed actions. To re-enable the analyzer, remove the label, then re-run all actions.

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⚠️ Note: The Code-Diff-Analyzer helps protect against potentially harmful code patterns. Please ensure you have thoroughly reviewed the changes beforehand.

Thanks.

[peterzhuamazon]

Expected bump and new added workflows.

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected