test(api): pin mirror-offset guide behavior; clarify guide end-default wording (#102)

- Add GetChannelGuideDataHandlerTests coverage for the mirror-channel path:
  asserts programme Start/Stop shift by PlayoutOffset and that the source
  channel's seeded PlayoutItem entities are not mutated, pinning the
  WithPlayoutOffset copy fix.
- ChannelController.GetGuide Swagger description said end defaults to "now
  plus XmltvDaysToBuild"; it actually defaults to start plus that window
  (matters when start is supplied). Reworded and regenerated v1.json.

ErsatzTV/Controllers/Api/ChannelController.cs

@@ -26,8 +26,8 @@ public class ChannelController(ChannelWriter<IBackgroundServiceRequest> workerCh
     [Tags("Channels")]
     [EndpointSummary("Get the JSON channel guide (EPG)")]
     [EndpointDescription(
-        "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now " +
-        "and end defaults to now plus the configured XmltvDaysToBuild window.")]
+        "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now, " +
+        "and end defaults to start plus the configured XmltvDaysToBuild window.")]
     [EndpointGroupName("general")]
     [ProducesResponseType(typeof(ChannelGuideResponseModel), StatusCodes.Status200OK)]
     public async Task<ChannelGuideResponseModel> GetGuide(