Add integration test for upgrades using download proxy

[michel-laterman]

What does this PR do?

Add an integration test that requires a download_proxy to be used.
This test will attempt to upgrade a fleet-managed agent, fail initially, add a download proxy and succeed in the upgrade.

Why is it important?

Untested workflow that has caused previous bugs.

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

N/A

Related issues

• Closes Add an upgrade test to confirm upgrades can succeed when a binary download proxy is configured #4322

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[michel-laterman]

I'm going to keep this as a draft until a new version is released

[elasticmachine]

⏳ Build in-progress, with failures

• Buildkite Build
• Commit: 5ad7bbd

Failed CI Steps

• check-ci
• Win2025:sudo:fleet
• arm:sudo: fleet

History

• 💔 Build #31435 failed bc73a30
• 💔 Build #31421 failed 9766995
• 💔 Build #31400 failed dd98949

cc @michel-laterman