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_cores/test_knowledge_manager.py

@@ -73,7 +73,6 @@ def sample_triples():
             id="test-doc-id",
             user="test-user",
             collection="default",  # This should be overridden
-            metadata=[]
         ),
         triples=[
             Triple(
@@ -93,7 +92,6 @@ def sample_graph_embeddings():
             id="test-doc-id",
             user="test-user",
             collection="default",  # This should be overridden
-            metadata=[]
         ),
         entities=[
             EntityEmbeddings(