[9.0](backport #7012) [flakiness]: List agents by hostname and policy id

[mergify]

What does this PR do?

This PR re-works how we extract an Agent status from the fleet api and fixes the current issue we have we paginated results.

Why is it important?

It fixes frequent flaky issues that make our CI red

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

How to test this PR locally

Related issues

• Relates [Flaky Test]: TestFleetAirGappedUpgradeUnprivileged – Agent status is not online #6968
• Relates [Flaky Test]: TestFleetAirGappedUpgradePrivileged – Agent status is not online #6924

---

This is an automatic backport of pull request #7012 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
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 47a209c

cc @pkoutsovasilis