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

trustgraph-flow/trustgraph/retrieval/graph_rag/rag.py

@@ -243,7 +243,6 @@ class Processor(FlowProcessor):
                 await flow("explainability").send(Triples(
                     metadata=Metadata(
                         id=explain_id,
-                        metadata=[],
                         user=v.user,
                         collection=v.collection,  # Store in user's collection, not separate explainability collection
                     ),