Failing to parse upgrade details should not be fatal to processing a checkin request

Follow up to https://github.com/elastic/fleet-server/pull/5176

Any error processing upgrade details is fatal to the checkin, which makes agent unable to get new configurations:

https://github.com/elastic/fleet-server/blob/496ee424b5ebc9bb495bee66f0845c683e45ea9b/internal/pkg/api/handleCheckin.go#L275-L279

Upgrade details are nice to have, not having them is not fatal. When this happens we should strip out the failed upgrade details and replace them with a sentinel value.

We should likely define an "INVALID" upgrade details state or similar to act as this sentinel value:

https://github.com/elastic/fleet-server/blob/496ee424b5ebc9bb495bee66f0845c683e45ea9b/model/openapi.yml#L358-L367