Removed deprecated subscriber types

We've simplified the SubscriberType enum by removing the following deprecated types:

• spammy — now consolidated into undeliverable with a specific undeliverability_reason
• malformed — now consolidated into undeliverable with undeliverability_reason of malformed
• disabled — this type has been removed entirely

This change streamlines subscriber management by using the existing undeliverability_reason field to provide more granular information about why a subscriber cannot receive emails, rather than having separate top-level types.

If you were filtering subscribers by subscriber_type=spammy, you should now filter by subscriber_type=undeliverable instead. The specific reason for undeliverability can be determined by checking the undeliverability_reason field.