[IMP] maintenance_service_http_monitoring: rework maintenance.request creation

Previously, a single ``maintenance.request`` was created per equipment,
regardless of how many services were down on that equipment. The name
was ``[HTTP KO] {equipment.name}`` and deduplication relied on a
name+date+equipment search that was fragile (manual clear of the field
would lose the reference to an existing open request).

This commit reworks the whole creation logic:

- **1 request per KO service** instead of 1 per equipment. Each failing
  ``service.instance`` gets its own ``maintenance.request``, allowing
  fine-grained tracking and independent resolution.

- **Request name** is now ``[HTTP KO] {service_url}``, making it
  immediately identifiable without opening the record.

- **Description** includes the error detail: ``HTTP {status_code}`` for
  HTTP errors, or a human-readable network error label when
  ``last_http_status_code == -1`` (timeout / DNS / SSL failures).

- **Deduplication** is now based solely on whether an open (non-done)
  ``maintenance.request`` already exists on the ``service.instance``
  via the new ``http_maintenance_request`` field. No date boundary —
  as long as the request is open, no new one is created.

- **``http_maintenance_request``** field moved from
  ``maintenance.equipment`` to ``service.instance``, where it belongs
  given the 1-request-per-service model. It is exposed as an optional
  hidden column in the service instance list view.

- ``_build_ko_services_description()`` is removed (no longer needed).

- ``create_http_maintenance_request()`` now receives a single
  ``service.instance`` recordset instead of a list.

Tests updated accordingly (14 tests total):
- Tests 2, 4, 9, 10, 12, 13 now assert on
  ``service_instance.http_maintenance_request``.
- Test 2 also verifies the request name contains the service URL and
  the description contains the HTTP status code.
- New test 14 asserts that two KO services on the same equipment
  produce two distinct requests with the correct names.

maintenance_service_http_monitoring/views/maintenance_equipment_views.xml

@@ -36,7 +36,6 @@
                         <field name="maintenance_mode" />
                         <field name="maintenance_mode_start" />
                         <field name="maintenance_mode_end" />
-                        <field name="http_maintenance_request" readonly="1" />
                     </group>
                 </page>
             </xpath>

maintenance_service_http_monitoring/views/service_instance_views.xml

@@ -17,6 +17,7 @@
                 <field name="last_http_check_date" />
                 <field name="last_http_status_code" />
                 <field name="http_status_ok" />
+                <field name="http_maintenance_request" optional="hide" />
             </field>
         </field>
     </record>