Update test scenarios for the new paginated GET /subscriptions response

[hdamker]

Problem description

code/Test_definitions/device-reachability-status-subscriptions-retrieveDeviceReachabilityStatusSubscriptionList.feature still tests the old, pre-0.9.0 contract for GET /subscriptions:

• The happy-path scenarios assert the response body complies with an array of OAS schema defined at "#/components/schemas/Subscription". Since #63, the response is a wrapper object (SubscriptionList: { subscriptions: [...], pagination: {...} }), not a bare array.
• No scenario exercises the new page / perPage query parameters.
• No error scenarios cover invalid page / perPage values, even though both parameters are documented as rejecting invalid values with a 400 error.

Expected action

Update the feature file so it matches the current OpenAPI definition:

• Change response-body assertions to check the SubscriptionList object (subscriptions array + pagination object), and add assertions for the X-Total-Count / X-Total-Pages / Link response headers.
• Add scenarios covering page and perPage, including at least one pagination-across-multiple-pages case.
• Add 400 error scenarios for invalid page / perPage values, matching the documented error behaviour on those parameters.

Additional context

Found during release review of r2.1 (PR #76). This is a test-only gap — the OpenAPI contract itself is sound. Not a blocker for r2.1; fix in a follow-up PR. Same gap, same root-cause PR pattern, also found in camaraproject/DeviceRoamingStatus#93.