Complete Phase 2: Domain Layer implementation

Updated ROADMAP.md to mark Phase 2 as completed and added detailed descriptions of completed tasks. Introduced three new value objects (`Base64String`, `TenantId`, and `PdfMetadata`) in the `DocumentOperator.Domain.Models.ValueObjects` namespace. These classes ensure type safety, immutability, and encapsulated validation.

- `Base64String`: Handles Base64 string creation, validation, and conversion.
- `TenantId`: Represents a tenant identifier with validation and normalization.
- `PdfMetadata`: Represents PDF metadata with computed properties.

Updated `DocumentOperator.Domain.csproj` to reflect the addition of these value objects. The project is now ready to begin Phase 3 (Infrastructure Layer).

DocumentOperator.Domain/DocumentOperator.Domain.csproj

@@ -9,7 +9,6 @@
   <ItemGroup>
     <Folder Include="Common\Results\" />
     <Folder Include="Constants\" />
-    <Folder Include="Models\ValueObjects\" />
   </ItemGroup>
 
 </Project>