[8.19](backport #5069) Include base error for json decode error responses

[mergify]

What is the problem this PR solves?

There is no information on why request bodies fail to decode as JSON

How does this PR solve the problem?

Include the base json decode error as part of the response.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• 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

---

This is an automatic backport of pull request #5069 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 966c4f8 has failed:

On branch mergify/bp/8.19/pr-5069
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 966c4f8.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/fragments/1750971846-Include-error-on-json-decode-error-responses.yaml
	modified:   internal/pkg/api/error.go
	modified:   internal/pkg/api/handleAck_test.go
	modified:   internal/pkg/api/handleAudit_test.go
	modified:   internal/pkg/api/handleCheckin_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/api/handleEnroll_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[prodsecmachine]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube