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/integration/test_object_extraction_integration.py

@@ -231,7 +231,6 @@ class TestObjectExtractionServiceIntegration:
             id="customer-doc-001",
             user="integration_test",
             collection="test_documents",
-            metadata=[]
         )
         
         chunk_text = """
@@ -299,7 +298,6 @@ class TestObjectExtractionServiceIntegration:
             id="product-doc-001",
             user="integration_test",
             collection="test_documents",
-            metadata=[]
         )
         
         chunk_text = """
@@ -373,7 +371,6 @@ class TestObjectExtractionServiceIntegration:
                 id=chunk_id,
                 user="concurrent_test",
                 collection="test_collection",
-                metadata=[]
             )
             chunk = Chunk(metadata=metadata, chunk=text.encode('utf-8'))
             chunks.append(chunk)
@@ -470,7 +467,7 @@ class TestObjectExtractionServiceIntegration:
         await processor.on_schema_config(integration_config, version=1)
         
         # Create test chunk
-        metadata = Metadata(id="error-test", user="test", collection="test", metadata=[])
+        metadata = Metadata(id="error-test", user="test", collection="test")
         chunk = Chunk(metadata=metadata, chunk=b"Some text that will fail to process")
         
         mock_msg = MagicMock()
@@ -507,7 +504,6 @@ class TestObjectExtractionServiceIntegration:
             id="metadata-test-chunk",
             user="test_user",
             collection="test_collection",
-            metadata=[]  # Could include source document metadata
         )
         
         chunk = Chunk(