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_storage/test_rows_cassandra_storage.py

@@ -190,7 +190,6 @@ class TestRowsCassandraStorageLogic:
                 id="test-001",
                 user="test_user",
                 collection="test_collection",
-                metadata=[]
             ),
             schema_name="test_schema",
             values=[{"id": "123", "value": "test_data"}],
@@ -252,7 +251,6 @@ class TestRowsCassandraStorageLogic:
                 id="test-001",
                 user="test_user",
                 collection="test_collection",
-                metadata=[]
             ),
             schema_name="multi_index_schema",
             values=[{"id": "123", "category": "electronics", "status": "active"}],
@@ -310,7 +308,6 @@ class TestRowsCassandraStorageBatchLogic:
                 id="batch-001",
                 user="test_user",
                 collection="batch_collection",
-                metadata=[]
             ),
             schema_name="batch_schema",
             values=[
@@ -365,7 +362,6 @@ class TestRowsCassandraStorageBatchLogic:
                 id="empty-001",
                 user="test_user",
                 collection="empty_collection",
-                metadata=[]
             ),
             schema_name="empty_schema",
             values=[],  # Empty batch