feat: Introduce NetEvolve.Pulse.Testing — FakeMediator for unit testing

[Copilot]

Lightweight fake IMediator implementation for unit testing without a DI container.

New package: NetEvolve.Pulse.Testing

• FakeMediator : IMediator — thread-safe (backed by ConcurrentDictionary), zero dependencies beyond NetEvolve.Pulse.Extensibility
• RequestSetup<TResponse> — fluent builder returned by SetupCommand / SetupQuery with .Returns(value) and .Throws(ex) / .Throws<T>()
• SetupEvent<TEvent>() — captures published events for later retrieval
• Verify<T>(int times) — asserts invocation count, throws descriptive InvalidOperationException on mismatch
• GetPublishedEvents<TEvent>() — returns captured events in publication order
• Unconfigured commands/queries throw InvalidOperationException with actionable message

Tests

• 21 tests in NetEvolve.Pulse.Testing.Tests.Unit covering all public API surface across net8.0, net9.0, net10.0

Usage

var mediator = new FakeMediator();

mediator.SetupCommand<CreateOrderCommand, OrderResult>()
    .Returns(new OrderResult { Id = "123" });

mediator.SetupEvent<OrderCreatedEvent>();

// exercise code under test...

mediator.Verify<CreateOrderCommand>(times: 1);
var events = mediator.GetPublishedEvents<OrderCreatedEvent>();

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