Add ValidatePDF feature with API, Swagger, and tests

Implemented the ValidatePDF feature end-to-end:
- Added `/api/v1/documents/validate` Minimal API endpoint.
- Introduced centralized ExceptionHandlingMiddleware.
- Configured Swagger with `AddSwaggerDocumentation` extension.
- Enabled XML comments in `DocumentOperator.API.csproj`.
- Updated DTOs with XML comments and added `FileSizeMB`.
- Added integration tests for the ValidatePDF endpoint (3 tests).
- Registered infrastructure services (e.g., `IPdfProcessor`).
- Refactored `Program.cs` to include middleware and endpoints.
- Updated PHASENPLAN.md and ROADMAP.md to reflect progress.
- Cleaned up code and made `Program` accessible for tests.

DocumentOperator.Tests/DocumentOperator.Tests.csproj

@@ -20,6 +20,7 @@
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.0" />
     <PackageReference Include="FluentAssertions" Version="7.0.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="8.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.3" />
@@ -34,6 +35,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <ProjectReference Include="..\DocumentOperator.API\DocumentOperator.API.csproj" />
     <ProjectReference Include="..\DocumentOperator.Domain\DocumentOperator.Domain.csproj" />
     <ProjectReference Include="..\DocumentOperator.Application\DocumentOperator.Application.csproj" />
     <ProjectReference Include="..\DocumentOperator.Infrastructure\DocumentOperator.Infrastructure.csproj" />