Phase 1: Replace GenericServices with Custom Services, MediatR, and FluentValidation

[devin-ai-integration]

Summary

This PR implements Phase 1 of the migration plan to replace GenericServices (not .NET Core compatible) with a modern architecture. It adds a new SampleWebApp.Core .NET Standard 2.0 project containing:

• Base classes: BaseDto, BaseQuery<T>, BaseCommand<T> for consistent patterns
• Result types: OperationResult, CreateResult, UpdateResult, DeleteResult, PagedResult<T>, and ISuccessOrErrors for backward compatibility
• DTOs: PostDto, SimplePostDto, TagDto, BlogDto with validation attributes
• Service interfaces: Generic (IListService, IDetailService, ICreateService, IUpdateService, IDeleteService) and domain-specific (IPostService, ITagService, IBlogService)
• MediatR handlers: Full CQRS implementation for Posts, Tags, and Blogs
• FluentValidation validators: PostValidator, TagValidator, BlogValidator
• Anti-corruption layer: GenericServicesAdapter to bridge new MediatR-based services with existing controller patterns
• DI configuration: ServiceCollectionExtensions.AddCoreServices()

Note: Package versions were adjusted from the spec to maintain .NET Standard 2.0 compatibility:

• MediatR: 10.0.1 → 9.0.0
• AutoMapper: 11.0.1 → 10.1.1
• FluentValidation: 11.1.0 → 10.4.0

Review & Testing Checklist for Human

• Verify package version changes are acceptable - The specified versions (MediatR 10.0.1, AutoMapper 11.0.1) require .NET Standard 2.1, so I downgraded to versions compatible with .NET Standard 2.0. Confirm this is acceptable for the migration path.
• Review repository interface design - Repository interfaces (IPostRepository, ITagRepository, IBlogRepository) return object types since the Core project doesn't reference DataLayer. Verify this approach works for Phase 2 integration.
• Validate business rules in PostValidator - Contains rules like "no ! in title", "can't end with ?", "can't mention sheep/lamb/cow/calf". Verify these match existing GenericServices validation behavior.
• Confirm GenericServicesAdapter scope - Currently only covers Post operations. Verify if Tag/Blog adapter methods are needed for Phase 2.
• Test build in CI - The project compiles locally with dotnet build but should be verified in CI.

Recommended test plan: Since this is Phase 1 (infrastructure only), verify the project builds successfully. Actual runtime testing will occur in Phase 2 when controllers are updated to use the new services.

Notes

• This PR does NOT modify existing controllers or views - that's Phase 2
• The existing GenericServices code remains untouched for gradual migration
• Runtime testing requires Windows/LocalDB which isn't available in this environment

Link to Devin run: https://app.devin.ai/sessions/b5a358f889ca47dc810452db58b86c5d
Requested by: Abhay Aggarwal (abhay.aggarwal@cognition.ai) / @abhay-codeium

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