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

[cmacknz]

Follow up to #5176

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

fleet-server/internal/pkg/api/handleCheckin.go

Lines 275 to 279 in 496ee42

	// Handle upgrade details for agents using the new 8.11 upgrade details field of the checkin.
	// Older agents will communicate any issues with upgrades via the Ack endpoint.
	if err := ct.processUpgradeDetails(r.Context(), agent, req.UpgradeDetails); err != nil {
	return fmt.Errorf("failed to update upgrade_details: %w", err)
	}

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:

fleet-server/model/openapi.yml

Lines 358 to 367 in 496ee42

	enum:
	- UPG_REQUESTED
	- UPG_SCHEDULED
	- UPG_DOWNLOADING
	- UPG_EXTRACTING
	- UPG_REPLACING
	- UPG_RESTARTING
	- UPG_WATCHING
	- UPG_ROLLBACK
	- UPG_FAILED