What does this PR do?

This PR includes the aggregated status of the agent node to the liveness health check.

As a bonus, it also adds status code assertion to the tests, which were missing before. (All liveness/readiness tests were passing without any assertions).

Why is it important?

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

Liveness probes will now fail if the configuration is invalid, likely causing the container to be restarted (see https://kubernetes.io/docs/concepts/configuration/liveness-readiness-startup-probes/#liveness-probe).

How to test this PR locally

1. Create an elastic-agent.yml file with an invalid output, i.e set use_output: nonexistent
2. Start elastic-agent with relevant monitoring endpoints enabled.
3. Verify that the agent is failed with elastic-agent status

┌─ fleet
│  └─ status: (STOPPED) Not enrolled into Fleet
└─ elastic-agent
   ├─ status: (FAILED) Invalid component model: failed to render components: invalid 'inputs.0.use_output', references an unknown output 'nonexistent'
   └─ info
      ├─ id: e1a1e08b-9b0c-4394-a024-d35b823d415b
      ├─ version: 9.2.0
      └─ commit: ff80471809aca1f2280ce55f0e24f85cefec5d55

4. Liveness probes should fail:

$ curl -w 'HTTP %{http_code}\n' 'http://localhost:6792/liveness?failon=degraded'
HTTP 500
$ curl -w 'HTTP %{http_code}\n' 'http://localhost:6792/liveness?failon=failed'
HTTP 500
$ curl -w 'HTTP %{http_code}\n' 'http://localhost:6792/liveness?failon=heartbeat'
HTTP 200

Related issues

• Closes Liveness endpoint does not consider overall agent state, only component state #9576