Summary

Nothing in the extension checks for @SoftDelete. A TIMESTAMP strategy therefore survives
metadata building and fails much later, while the SessionFactory pre-generates mutation statements:
the strategy produces a ColumnWriteFragment whose fragment is null, and
AbstractMqlTranslator.visitColumnWriteFragment passes it to a regex matcher.

Reproducer

@Entity @Table(name = "items")
@SoftDelete(strategy = SoftDeleteType.TIMESTAMP)
static class ItemSoftDeletedByTimestamp {
    @Id int id;
}

java.lang.NullPointerException: Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null
  at java.base/java.util.regex.Pattern.matcher(Pattern.java:1145)
  at com.mongodb.hibernate.internal.translate.AbstractMqlTranslator.visitColumnWriteFragment(AbstractMqlTranslator.java:406)

Proposed fix

Decide whether the ACTIVE and DELETED strategies are supported, then add a boot-time rule in
MongoAdditionalMappingContributor that rejects the strategies that are not, so the failure lands
at boot with a message naming the annotation.

The zero-parameter branch of visitColumnWriteFragment already throws
FeatureNotSupportedException for a fragment it cannot parse. Extending that branch to cover a
null fragment is worth doing regardless, as a backstop for any other source of a null fragment.

Acceptance

The mapping is rejected with FeatureNotSupportedException whose message names @SoftDelete and
the offending strategy, and the supported strategies, if any, keep working.