feat(api): add route names and handler tests for picker list endpoints (#105)

The WIP GET /api/filler-presets, /api/watermarks, and
/api/graphics-elements endpoints omitted the Name= route attribute
used by the other API GET actions (e.g. FFmpegProfileController). Add
Name = "GetFillerPresets" / "GetWatermarks" / "GetGraphicsElements" for
consistency.

Also add handler tests (FillerPresetHandlerTests, WatermarkHandlerTests,
GraphicsElementHandlerTests) covering the GetAll*ForApi handlers using
the InMemoryTvContext harness: multi-row projection to (Id, Name) DTOs,
the empty-DB case, and the graphics-element ordering rule (named
elements sort before elements whose projected Name equals FileName).

ErsatzTV/Controllers/Api/FillerPresetController.cs

@@ -9,7 +9,7 @@ namespace ErsatzTV.Controllers.Api;
 [ApiController]
 public class FillerPresetController(IMediator mediator) : ControllerBase
 {
-    [HttpGet("/api/filler-presets")]
+    [HttpGet("/api/filler-presets", Name = "GetFillerPresets")]
     [Tags("Filler Presets")]
     [EndpointSummary("Get all filler presets")]
     [EndpointGroupName("general")]