Adds automated API tests for the Petstore application, including fixtures, schema validation, negative scenarios, and a full end‑to‑end workflow test for PATCH /store/order/{order_id}. This PR improves test coverage, validates core functionality, and ensures consistent behavior across pet creation, order placement, and order status updates

Evidence:

Defect Summary:

• Invalid status error message uses a literal placeholder ({status}) instead of the actual value.
• POST /pets accepts incomplete or invalid payloads due to missing required‑field validation.
• Duplicate pet ID check fails when IDs are provided as strings vs integers.
• GET /pets/{id} returns an HTML 404 page for invalid IDs instead of a JSON error response.
• PATCH /store/order writes the new status before validating it, causing invalid data to be stored.
• PATCH /store/order throws a KeyError when the status field is missing in the request body.
• POST /store/order allows requests without pet_id, resulting in misleading error messages.
• Pets can be re‑ordered after status transitions because no duplicate‑order prevention exists.
• findByStatus returns 200 with an empty list even when no pets match the requested status.
• PATCH /store/order returns only a success message instead of the updated order or pet details.