fix: resolve all mypy type errors

tests/test_paste_management.py

@@ -95,7 +95,9 @@ def create_paste(
 
     response = client.post("/", data=content, headers=headers)
     assert response.status_code == 201
-    return response.get_json()["id"]
+    data = response.get_json()
+    assert data is not None
+    return str(data["id"])
 
 
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