Add emulator handler for ForwardRequest with mock response store

[Mielek]

Summary

Implement ForwardRequestHandler with mock backend response store for testing full request pipelines.

How It Works

ForwardRequest is the policy that sends the request to the backend service. In the emulator, we can't make real HTTP calls, so users pre-register mock responses via SetupForwardRequest().

Usage

var test = new MyDocument().AsTestDocument();

// Default response for all ForwardRequest calls
test.SetupForwardRequest().ReturnsDefault(new MockBackendResponse
{
    StatusCode = 200,
    Body = """{"result": "ok"}""",
    Headers = { { "Content-Type", ["application/json"] } }
});

test.RunBackend();
test.Context.Response.Body.Content.Should().Be("""{"result": "ok"}""");

// Sequential responses (queue-based)
test.SetupForwardRequest()
    .Returns(new MockBackendResponse { StatusCode = 200, Body = "first" })
    .Returns(new MockBackendResponse { StatusCode = 201, Body = "second" });

New Infrastructure

• ForwardRequestStore — queue-based response store with Returns() and ReturnsDefault()
• MockBackendResponse — record with StatusCode, StatusReason, Headers, Body
• SetupForwardRequest() — extension on TestDocument

Changes

• Modified: ForwardRequestHandler.cs — applies mock response from store
• Modified: ForwardRequestTests.cs — 6 test cases (was 1)
• Modified: GatewayContext.cs, TestDocumentExtensions.cs
• New: ForwardRequestStore.cs, MockBackendResponse.cs

Test Coverage

Test	Description
DefaultResponse	No mock setup, default 200 OK
MockedResponse	Full mock with status, body, headers
SequentialResponses	Queue-based responses consumed in order
ErrorResponse	500 error simulation
WithConfig	ForwardRequest with config (timeout)
Callback	Callback override

168 tests pass, 0 regressions.