Add comprehensive integration tests for AutoCrudAdmin

[Minkov]

Implements a complete integration test suite covering:

• CRUD operations (Create, Read, Update, Delete)
• Authorization filters (sync and async)
• Form generation and validation
• Autocomplete functionality
• Grid configuration and rendering
• Helper methods and reflection utilities

Test Infrastructure:

• TestDbContext with in-memory database
• DatabaseFixture for test data seeding
• TestWebApplicationFactory for integration testing
• Mock objects for isolation

Coverage includes:

• 60+ test cases across 6 test files
• Tests for happy paths and error scenarios
• Validation of form controls and grid columns
• Authorization success and failure cases
• Primary key detection (single and composite)
• Type extension methods

All tests use xUnit, FluentAssertions, and Moq for
maintainable and readable test code. Tests run in
isolation using in-memory database.

Related to issue: Initial test coverage for the library