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/conftest.py

@@ -28,7 +28,6 @@ def sample_text_document():
     """Sample document with moderate length text."""
     metadata = Metadata(
         id="test-doc-1",
-        metadata=[],
         user="test-user",
         collection="test-collection"
     )
@@ -44,7 +43,6 @@ def long_text_document():
     """Long document for testing multiple chunks."""
     metadata = Metadata(
         id="test-doc-long",
-        metadata=[],
         user="test-user",
         collection="test-collection"
     )
@@ -61,7 +59,6 @@ def unicode_text_document():
     """Document with various unicode characters."""
     metadata = Metadata(
         id="test-doc-unicode",
-        metadata=[],
         user="test-user",
         collection="test-collection"
     )
@@ -87,7 +84,6 @@ def empty_text_document():
     """Empty document for edge case testing."""
     metadata = Metadata(
         id="test-doc-empty",
-        metadata=[],
         user="test-user",
         collection="test-collection"
     )