[fast checkin]: Work around the config limitations

[moukoublen]

What does this PR do?

Because of this bug we are unable to enable the fast checkin in agentless. Here is a workaround that can be backported to 9.2, to overcome the original root cause.

The already implemented tests cover this change.

Why is it important?

Due to the config bug we cannot enable to fast check-in in agentless through the configuration file.

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

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[elasticmachine]

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

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 612c10e

Failed CI Steps

• Win2025:sudo:fleet
• x86_64:sudo: default
• x86_64:sudo: fleet - platform-ingest-elastic-agent-debian-11-1762801856

cc @moukoublen