refactor: simplify test cases with proper AutoFixture and NSubstitute usage

[j-d-ha]

Description

Clean up and simplify test cases across the codebase to make better use of AutoFixture and NSubstitute with the [AutoNSubstituteData] attribute pattern.

Objectives

• Consolidate test setup using [AutoNSubstituteData] where appropriate
• Remove manual fixture and mock creation where AutoFixture can generate test data
• Simplify test logic by leveraging NSubstitute integration
• Reduce boilerplate and improve readability

Scope

• Review existing test files across all unit test projects
• Identify opportunities to simplify using the AutoNSubstituteData pattern
• Update tests to follow the established patterns in CLAUDE.md

Notes

• Refer to CLAUDE.md AutoNSubstituteData pattern documentation
• Use [Frozen] attribute on mocks that need assertion verification
• Use manual Fixture helper only when mocks need complex SetupDefaults() configuration