Ensure idempotent dependency registration for MediatorKit

[coderynx]

This pull request updates the dependency injection logic in MediatorKit to prevent duplicate service registrations and avoid overwriting existing registrations for core interfaces like ISender and IPublisher. It replaces AddScoped with TryAddScoped and TryAddEnumerable to ensure that services are only registered if they haven't already been added, improving compatibility and reliability when integrating with other libraries or when configuring services multiple times. The tests have been expanded to verify these behaviors.

Dependency injection improvements:

• Replaced AddScoped with TryAddScoped for core services (ISender, IPublisher) in DependencyInjection.cs to prevent overwriting existing registrations.
• Updated registration of pipeline behaviors and handlers to use TryAddEnumerable or TryAddScoped, ensuring duplicate registrations are avoided when adding handlers or behaviors multiple times. [1] [2] [3] [4] [5] [6]

Testing enhancements:

• Added unit tests to verify that AddMediatorKit does not overwrite existing ISender or IPublisher registrations. [1] [2]
• Added tests to ensure that request and notification handlers are not registered multiple times when AddMediatorKit is called repeatedly. [1] [2]
• Introduced custom sender and publisher classes in test files to facilitate these new tests. [1] [2]

Version update:

• Bumped package version from 1.8.0 to 1.9.0 in version.json.