Feat/add tests infrastructure

[PureJoyMind]

Summary

This PR sets up the initial test infrastructure for FastCrud and introduces the first set of unit tests.

Linked issue: #4

Changes

• Test infrastructure

  • Added test dependencies to Directory.Packages.props so test projects follow solution defined packages
  • Created a test folder in solution folder.
  • Used the naming convention "<ProjectName.Tests.TestType>". For example the unit test project for Core is named: "FastCrud.Core.Tests.Unit"

• Test models & DTOs: Decided to use the Customer and Order models and their Dtos from the sample api project, but copied them and created TestCustomer etc.

Unit tests (currently - changes through time)

• FastCrud.Core.Tests.Unit project covers CrudService<> type's methods. Each method has it's own test class.

Roadmap

• Extend unit test coverage for other CRUD operations

Notes

• Opened as a Draft PR to allow discussion on scope (unit tests only vs. integration tests included here) and while I keep adding tests until the requirements are satisfied

[ashkanRmk]

Thanks a lot for kicking this off! The structure and naming look good and aligns with what we discussed. I’ll go through the details and leave any review notes by the weekend. Feel free to keep pushing more unit tests in the meantime.

[ashkanRmk]

Heads-up: I just pushed some updates to the dev branch (structural tweaks + minor service changes), so please pull/rebase from dev and resolve any merge conflicts before I start the review.

[PureJoyMind]

I'll fetch from dev and continue writing tests! Thanks for assigning me. We'll discuss any changes you'd like to me to make whenever you review the tests that I've written in the mean time and we'll be in touch.