Rebrand MediatR to Mediator in code and docs

Renamed all classes, methods, parameters, and XML documentation from "MediatR" to "Mediator" across the codebase. This includes types like MediatRServiceConfiguration (now MediatorServiceConfiguration) and methods such as AddMediatR (now AddMediator). No functional changes were made; this is a systematic rebranding for clarity and consistency.

Author: Vadim Belov