Add comprehensive tests to improve code coverage for missing lines

[Copilot]

This PR addresses the code coverage gaps identified in the Codecov report, where patch coverage was only 57.69% with 11 lines missing coverage across 4 files.

Coverage Improvements

SlimBusEventPublisher.cs - Added integration tests covering:

• Event publishing with IEventItem objects and header propagation from AdditionalData
• Event publishing with regular objects (non-IEventItem path)
• Cancellation token handling for both scenarios
• Empty AdditionalData dictionary edge case

EventExtensions.cs - Added unit tests covering:

• GetEntityKeyValues method with entities that have primary keys
• Key-value extraction and dictionary creation logic

IEventItem.cs - Added comprehensive tests for the EventItem record covering:

• EventType property behavior and automatic type name resolution
• AdditionalData dictionary initialization and case-insensitive behavior
• Multiple entries, null value handling, and dictionary operations

IHook.cs - Added tests for the HookAsync abstract base class covering:

• Default implementations of RunBeforeSaveAsync and RunAfterSaveAsync
• Cancellation token support in both hook methods
• Base class functionality when inherited by concrete implementations

Test Quality

All tests follow the existing project patterns using xUnit and Shouldly, with integration tests using real services where appropriate rather than excessive mocking. The tests target the specific missing lines identified in the coverage report while maintaining the project's high standards for test quality.

Fixes #117.

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