Add explicit model types to Content/Part editor views

[okalangkenneth]

Closes #19273

Introduces a strongly-typed ContentPartShapeViewModel for the ContentPart_Edit
editor shape and adds explicit @model directives to the remaining Content/Part
views in OrchardCore.Contents, continuing the pattern established in #18641.

Changes

• ContentPartShapeViewModel: new class in OrchardCore.ContentManagement.Display/ViewModels/,
  extending ShapeViewModel with typed ContentPart and ContentTypePartDefinition properties
• ContentItemDisplayCoordinator : CreateShapeResult() now instantiates
  ContentPartShapeViewModel directly; BuildEditorAsync and UpdateEditorAsync
  assign typed properties instead of Properties[...] dictionary entries. A fallback
  else branch preserves the old Properties[...] assignment for any code path that
  produces a non-ContentPartShapeViewModel shape
• ContentPart.Edit.cshtml : @model ContentPartShapeViewModel added; all member
  access is now statically typed
• ContentItem.cshtml : @model IShape added (Items is declared on IShape)
• Stereotype.Edit.cshtml, Stereotype.DetailAdmin.cshtml, Stereotype.Summary.cshtml,
  Stereotype.SummaryAdmin.cshtml: @model IShape added

Notes

• Shape type registration is unaffected: ShapeResult.BuildAndAddShapeAsync always
  sets Metadata.Type from the shape type string, regardless of how the factory creates
  the shape
• The display-path ContentPart.cshtml variants (and Content.cshtml) are not included
  here, those shapes are ZoneHolding instances, and the zone surface requires a
  separate approach; deferred as a follow-up

[gvkries]

The ShapeResult is expecting this to be a Shape instance, otherwise the editors are not added to the parentShape here:

OrchardCore/src/OrchardCore/OrchardCore.DisplayManagement/Views/ShapeResult.cs

Lines 473 to 476 in 008014e

	if (parentShape is Shape shape)
	{
	await shape.AddAsync(newShape, position);
	}

I'm not sure if that's a bug in ShapeResult or by design.

[okalangkenneth]

It is a real regression. ShapeViewModel doesn't inherit Shape so the is Shape check at ShapeResult.cs:473 would silently discard child shapes when ContentPartShapeViewModel is the terminal parent in zone traversal. I have fixed it by replacing the guard with a null-check....parentShape is typed IShape at that point and IShape contracts AddAsync so no cast is needed.