fix(api): serialize pipeline file-upload created_at (#32098)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/tests/unit_tests/controllers/service_api/dataset/test_rag_pipeline_file_upload_serialization.py

@@ -0,0 +1,62 @@
+"""
+Unit tests for Service API knowledge pipeline file-upload serialization.
+"""
+
+import importlib.util
+from datetime import UTC, datetime
+from pathlib import Path
+
+
+class FakeUploadFile:
+    id: str
+    name: str
+    size: int
+    extension: str
+    mime_type: str
+    created_by: str
+    created_at: datetime | None
+
+
+def _load_serialize_upload_file():
+    api_dir = Path(__file__).resolve().parents[5]
+    serializers_path = api_dir / "controllers" / "service_api" / "dataset" / "rag_pipeline" / "serializers.py"
+
+    spec = importlib.util.spec_from_file_location("rag_pipeline_serializers", serializers_path)
+    assert spec
+    assert spec.loader
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)  # type: ignore[attr-defined]
+    return module.serialize_upload_file
+
+
+def test_file_upload_created_at_is_isoformat_string():
+    serialize_upload_file = _load_serialize_upload_file()
+
+    created_at = datetime(2026, 2, 8, 12, 0, 0, tzinfo=UTC)
+    upload_file = FakeUploadFile()
+    upload_file.id = "file-1"
+    upload_file.name = "test.pdf"
+    upload_file.size = 123
+    upload_file.extension = "pdf"
+    upload_file.mime_type = "application/pdf"
+    upload_file.created_by = "account-1"
+    upload_file.created_at = created_at
+
+    result = serialize_upload_file(upload_file)
+    assert result["created_at"] == created_at.isoformat()
+
+
+def test_file_upload_created_at_none_serializes_to_null():
+    serialize_upload_file = _load_serialize_upload_file()
+
+    upload_file = FakeUploadFile()
+    upload_file.id = "file-1"
+    upload_file.name = "test.pdf"
+    upload_file.size = 123
+    upload_file.extension = "pdf"
+    upload_file.mime_type = "application/pdf"
+    upload_file.created_by = "account-1"
+    upload_file.created_at = None
+
+    result = serialize_upload_file(upload_file)
+    assert result["created_at"] is None