Do not enforce minimum value of page_number at model level

- This is to avoid spurious constraints being added to the OpenAPI
  schema
- Validate page number dynamically for database queries
- Add test for `page_number` validation

openapi/index_openapi.json

@@ -199,7 +199,6 @@
             "required": false,
             "schema": {
               "title": "Page Number",
-              "minimum": 1.0,
               "type": "integer",
               "description": "RECOMMENDED for use with _page-based_ pagination: using `page_number` and `page_limit` is RECOMMENDED.\nIt is RECOMMENDED that the first page has number 1, i.e., that `page_number` is 1-based.\nExample: Fetch page 2 of up to 50 structures per page: `/structures?page_number=2&page_limit=50`."
             },
@@ -578,8 +577,8 @@
             "type": "string",
             "description": "Date and time representing when the entry was last modified.\n\n- **Type**: timestamp.\n\n- **Requirements/Conventions**:\n    - **Support**: SHOULD be supported by all implementations, i.e., SHOULD NOT be `null`.\n    - **Query**: MUST be a queryable property with support for all mandatory filter features.\n    - **Response**: REQUIRED in the response unless the query parameter `response_fields` is present and does not include this property.\n\n- **Example**:\n    - As part of JSON response format: `\"2007-04-05T14:30:20Z\"` (i.e., encoded as an [RFC 3339 Internet Date/Time Format](https://tools.ietf.org/html/rfc3339#section-5.6) string.)",
             "format": "date-time",
-            "x-optimade-queryable": "must",
-            "x-optimade-support": "should"
+            "x-optimade-support": "should",
+            "x-optimade-queryable": "must"
           }
         },
         "description": "Contains key-value pairs representing the entry's properties."