[8.19] (backport #8670) [testing] validate artifact hashes in artifact fetcher

[mergify]

What does this PR do?

This PR improves the robustness of the test artifact fetcher by validating the integrity of downloaded packages and introducing retry logic for both corrupted downloads and snapshot metadata fetch failures. Specifically, it:

• Validates the .sha512 checksum of each downloaded artifact to detect corruption.
  • Automatically retries corrupted downloads, up to 3 attempts, using a constant backoff of 3 seconds.
• Applies retry logic to snapshot metadata resolution (e.g., .json file for snapshot build IDs), which previously failed with transient errors.

Why is it important?

This PR addresses two known failure cases in upgrade test pipelines:

• Corrupted downloads: Previously, corrupted .tar.gz artifacts would not be detected, leading to tar extraction errors such as flate: corrupt input before offset.... This caused test flakiness and debugging overhead.
• Transient 502 errors during metadata fetch: Snapshot builds occasionally return 502 errors when fetching the latest snapshot manifest. Without retry logic, these tests fail unnecessarily.

By introducing checksum validation and retry logic, this PR makes upgrade tests more reliable and deterministic.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

None. This change only affects internal test infrastructure and does not impact users of the Elastic Agent or its public APIs.

How to test this PR locally

mage unitTest

Related issues

• Closes Upgrade test artifact fetcher does not validate checksums or retry corrupted downloads. #8634
• Closes to [Flaky Test]: TestFleetManagedUpgradeUnprivileged – failed to find artifact URI for version 9.0.0-SNAPSHOT: failed to find snapshot information for version "9.0.0-SNAPSHOT" #7853

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This is an automatic backport of pull request #8670 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube