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_chunking/test_recursive_chunker.py

@@ -184,7 +184,6 @@ class TestRecursiveChunkerSimple(IsolatedAsyncioTestCase):
         mock_text_doc = MagicMock()
         mock_text_doc.metadata = Metadata(
             id="test-doc-123",
-            metadata=[],
             user="test-user",
             collection="test-collection"
         )