Add support for 'offset' query parameter in Search API Decoupled endpoint pagination

Problem/Motivation

This feature enhances the pagination capabilities of the Search API Decoupled module by adding support for an explicit offset query parameter in API requests.
It allows clients to skip an arbitrary number of results, enabling more flexible data fetching and integration with frontend applications that use infinite scrolling or custom pagination strategies.

Currently, the prepareQuery() method in SearchApiEndpointController calculates result offsets based solely on the page parameter:

'offset' => $request->get('page') !== NULL ? $request->get('page') * $limit : 0,

This works well for traditional pagination (page=0,1,2...), but doesn’t support skipping a specific number of items independent of pages — a common need for:

• Infinite scrolling UIs (React, Vue, Next.js, etc.)
• Mobile apps requesting partial data loads
• Data exports or analytics queries requiring custom start points

By introducing an offset parameter, developers can now control the exact starting point of search results without depending on page calculations.

Proposed resolution

In SearchApiEndpointController::prepareQuery(), replace the existing pagination logic with this improved version:

// Determine pagination and offset.
$page = $request->get('page');
$offset_param = $request->get('offset');
$offset = 0;

// If both 'page' and 'offset' are present, combine them.
if ($page !== NULL && is_numeric($page)) {
  $offset = (int) $page * $limit;
  if ($offset_param !== NULL && is_numeric($offset_param)) {
    $offset += (int) $offset_param;
  }
}
// If only 'offset' is provided, use it directly.
elseif ($offset_param !== NULL && is_numeric($offset_param)) {
  $offset = (int) $offset_param;
}

// Prevent negative offset values.
$offset = max(0, $offset);

// Build query with computed limit and offset.
$query = $search_api_index->query([
  'limit' => $limit,
  'offset' => $offset,
]);

Example Responses

✅ Example Request 1:
GET /api/search/experiences?offset=4

Effect: Returns results starting from the 5th item.

✅ Example Request 2:
GET /api/search/experiences?page=1&offset=2

Effect: Returns results starting from the 7th item (assuming a limit of 4 → 1 * 4 + 2 = 6 → item #7).