Remove redundant metadata (#685)

The metadata field (list of triples) in the pipeline Metadata class
was redundant. Document metadata triples already flow directly from
librarian to triple-store via emit_document_provenance() - they don't
need to pass through the extraction pipeline.

Additionally, chunker and PDF decoder were overwriting metadata to []
anyway, so any metadata passed through the pipeline was being
discarded.

Changes:
- Remove metadata field from Metadata dataclass
  (schema/core/metadata.py)
- Update all Metadata instantiations to remove metadata=[]
  parameter
- Remove metadata handling from translators (document_loading,
  knowledge)
- Remove metadata consumption from extractors (ontology, agent)
- Update gateway serializers and import handlers
- Update all unit, integration, and contract tests

tests/unit/test_knowledge_graph/conftest.py

@@ -29,11 +29,10 @@ class Triple:
         self.o = o
 
 class Metadata:
-    def __init__(self, id, user, collection, metadata):
+    def __init__(self, id, user, collection):
         self.id = id
         self.user = user
         self.collection = collection
-        self.metadata = metadata
 
 class Triples:
     def __init__(self, metadata, triples):
@@ -110,7 +109,6 @@ def sample_triples(sample_triple):
         id="test-doc-123",
         user="test_user",
         collection="test_collection",
-        metadata=[]
     )
     
     return Triples(
@@ -126,7 +124,6 @@ def sample_chunk():
         id="test-chunk-456",
         user="test_user",
         collection="test_collection",
-        metadata=[]
     )
     
     return Chunk(