Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.API/Middleware/TenantResolutionMiddleware.cs

@@ -0,0 +1,9 @@
+namespace DocumentService.API.Middleware
+{
+    /// <summary>
+    /// Placeholder middleware for multi-tenancy resolution via X-API-Key header.
+    /// </summary>
+    public class TenantResolutionMiddleware
+    {
+    }
+}