fix upload file size contract (#3408)

backend/tests/test_client.py

@@ -1472,6 +1472,7 @@ class TestUploads:
             assert result["success"] is True
             assert len(result["files"]) == 1
             assert result["files"][0]["filename"] == "test.txt"
+            assert result["files"][0]["size"] == len("hello")
             assert "artifact_url" in result["files"][0]
             assert "message" in result
             assert (uploads_dir / "test.txt").exists()
@@ -1551,6 +1552,8 @@ class TestUploads:
             assert len(result["files"]) == 2
             names = {f["filename"] for f in result["files"]}
             assert names == {"a.txt", "b.txt"}
+            sizes = {f["filename"]: f["size"] for f in result["files"]}
+            assert sizes == {"a.txt": 1, "b.txt": 2}
             # Verify artifact_url is present
             for f in result["files"]:
                 assert "artifact_url" in f
@@ -2458,6 +2461,7 @@ class TestGatewayConformance:
         parsed = UploadResponse(**result)
         assert parsed.success is True
         assert len(parsed.files) == 1
+        assert parsed.files[0].size == len("hello")
 
     def test_get_memory_config(self, client):
         mem_cfg = MagicMock()