chore: make short tests faster (#810)

The test cases covering failure behavior were all inadvertently using
exponential backoff in test runs, causing unnecessary slowdowns. This
commit disables exponential backoff in tests by having the mock API
server return 400 status codes instead of 501s when a request is not
recognized. As a result, the exponential backoff logic does not trigger.

This change reduces the time taken by ~75% to about ~5s.

internal/mock/sqladmin.go

@@ -47,7 +47,7 @@ func httpClient(requests ...*Request) (*http.Client, string, func() error) {
 				}
 			}
 			// Unexpected requests should throw an error
-			resp.WriteHeader(http.StatusNotImplemented)
+			resp.WriteHeader(http.StatusBadRequest)
 			// TODO: follow error format better?
 			resp.Write([]byte(fmt.Sprintf("unexpected request sent to mock client: %v", req)))
 		},