[Fleet] Add graceful error handling for invalid sortField on GET api/fleet/agents

[Supplementing]

Summary

Closes https://github.com/elastic/ingest-dev/issues/5992

• Added error handling when an invalid sortField query field is passed to the GET api/fleet/agents endpoint. This error wrapping will handle other ES client errors and return the root cause as a 400 (bad request) response letting the user know what was wrong, instead of just returning a 500 error.
• Also added additional test coverage to verify the handler will throw/succeed when its supposed to

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• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
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[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[Supplementing]

@elasticmachine merge upstream

[elasticmachine]

💚 Build Succeeded

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• Commit: 8eb0c07

Metrics [docs]

✅ unchanged

[jen-huang]

I'm not loving this approach. it's extremely specific to this Fleet API & this type of ES error.

I can see from testing a direct ES query that this kind of error already returns 400 from ES. example curl:

curl -X GET -H "Content-Type: application/json" -d '{"sort":[{"invalid_field":{"order":"desc"}}]}' "http://elastic:changeme@localhost:9200/.kibana_ingest/_search" -v

can we explore a solution where the status code & error is just passed directly from ES? even better if we can handle it generically for all agent APIs (since they access ES directly instead of with soClient)