fix(ci): support Dependabot in external PR test workflow

[vishal-bala]

Summary

• Allow the manual external PR test workflow to run against Dependabot PRs in addition to fork-based PRs
• Record the resolved PR number once and reuse it in check-run messages, rename the check/workflow labels to cover all external PRs, and disable persisted checkout credentials during execution

With this change, a maintainer can go to Actions -> Test External PR, provide the PR number of the Dependabot PR, and run the workflow. This will create a status check on the PR that resolves to whether the tests have passed.

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Note

Low Risk
Low risk CI-only change; main impact is whether the workflow correctly validates/labels external PRs and reports check-run status for the intended PR/commit.

Overview
Renames the manual workflow from fork-only to external PR testing and broadens validation to allow Dependabot PRs in addition to forked PRs.

Resolves and reuses the PR number via job outputs, improving check-run messaging, and enhances check runs with a details_url plus richer status text. Checkout now disables persisted credentials (persist-credentials: false) when pulling external PR code.

^{Reviewed by Cursor Bugbot for commit 4d9bbf7. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[github-actions]

Review completed. The changes properly extend the workflow to support Dependabot PRs while maintaining security by preserving the manual trigger requirement and disabling persisted credentials.

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🤖 Automated review complete. Please react with 👍 or 👎 on the individual review comments to provide feedback on their usefulness.

[github-actions]

Reviewed the workflow changes for Dependabot support. Found a potential issue with null reference handling that could cause the workflow to fail. See inline comment for details.

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🤖 Automated review complete. Please react with 👍 or 👎 on the individual review comments to provide feedback on their usefulness.

[nkanu17]

👍