malformed docs in bulk requests interfere with normal operations

[michel-laterman]

fleet-server makes a few different bulk reads where all documents retrieved are immediately parsed to structs defined in model/schema.json. For example, when we query for the latest policies or handling actions.

If a single document fails to unmarshal correctly, we return nil, err even if it's a single doc out of a large set.
This will stop the fleet-server from processing policies or actions, which could result in agents going offline.
For example, an agent with the policy A checks in, policy A is queried and parsed correctly, yet policy B fails to parse due to an incorrect attribute, fleet-server will return a 503 error to the agent.

We should change how bulk returns are handled to either:

1. log and ignore the parsing error
   or
2. return the parsed hits with any errors (using errors.Join) and have the callers handle these cases