WS-601: addresses bug in additional_metadata section of image manifest

This MR addresses bug in additional_metadata section of manifest that has a class repr rather than a JSON-serializable value

• tests dump all JSON strings; if dumps succeed, the JSON is good
• removed contrived test case
• added test case based on what an image ingestion staging dir is likely to contain

TODO next MR: implement additional tests for new image ingestion test case

apps/cli/executables/pexable/ingest_envoy/ingest_envoy/ingestion_manifest.py

@@ -238,7 +238,7 @@ class IngestionManifestBuilder:
 
         artifacts_file = self.staging_source_dir / self._build_artifacts_filename()
         artifacts_ap = AncillaryProduct(
-            type=AncillaryProductType.INGESTION_ARTIFACTS, filename=str(artifacts_file)
+            type=AncillaryProductType.INGESTION_ARTIFACTS, filename=artifacts_file.name
         )
         manifest.output_group.ancillary_products.append(artifacts_ap)
         self.write_ingestion_artifacts_tar(artifacts_file)