Require spec-mandatory relation endpoints, dedupe serializer namespace bootstrap, and drop ProvNamespace::contains()

[amateescu]

Changes

• Builder methods now require every PROV-mandatory relation endpoint as a non-nullable parameter (via resolveRequiredIdentifier()): wasGeneratedBy/wasInvalidatedBy entity; used/wasStartedBy/wasEndedBy/wasAssociatedWith activity; wasInformedBy both activities; wasDerivedFrom (and wasRevisionOf/wasQuotedFrom/hadPrimarySource) both entities; wasAttributedTo entity+agent; actedOnBehalfOf both agents; wasInfluencedBy both; specializationOf/alternateOf both; hadMember collection+entity; mentionOf specific+general+bundle; the dictionary relations' ref params. A builder call can no longer emit a - marker in a grammar-mandatory slot.
• Model constructors are non-nullable, and the PROV-N/JSON/XML deserializers throw DeserializationException, only for endpoints no real document omits: Generation/Invalidation entity, Specialization/Alternate/Mention entity pairs, and the dictionary refs. The rest stay nullable and lenient on the read path so the openprov/testcases corpus (whose reference implementation emits - in some mandatory slots, e.g. wasAttributedTo agent and mentionOf bundle) still round-trips.
• Mandatory/optional split re-derived from the PROV-N grammar productions, with mentionOf from PROV-LINKS and the dictionary relations from PROV-DICTIONARY.
• identifier loses its constructor default on the affected relations (a required parameter cannot follow an optional one); it stays nullable-typed but is no longer positionally omittable.
• NamespaceManager::forContainer() and stripDefaultSentinel() replace the namespace-bootstrap loop duplicated across the serializers, deserializers, and ProvGraph, and the three copies of the default-sentinel strip.
• Remove ProvNamespace::contains() (no callers; NamespaceManager owns URI matching).
• Add RecordBuilder::getNamespaceManager() so a caller assembling attributes can share the builder's live registry.