Expand Request Schema Validation

The codebase is currently missing schema validation for inputs and outputs. Before sending a request to the Kroger API, we should be able to identify whether the request content is valid, and provide helpful errors/feedback to the user when it is not.

Start by tracing how the Python client constructs requests to and handles responses from the Kroger API; no file or test is named in the issue. Define the request and response validation scope and error expectations, then verify invalid inputs and outputs produce helpful feedback before considering the work done.

Written by the indexing model from the issue text.