Introduce IHttpRequestHead to decouple public API from Kayak

[Copilot]

Kayak.Http.HttpRequestHead was leaking into every public interface and internal method signature, coupling the entire HttpMock API to the Kayak library.

New types

• IHttpRequestHead — HttpMock-owned interface exposing Method, Uri, Headers, and HasBody()
• KayakHttpRequestHeadAdapter — thin adapter wrapping Kayak.Http.HttpRequestHead, constructed once at the Kayak boundary

Boundary isolation

RequestProcessor.OnRequest is the sole Kayak entry-point (required by IHttpRequestDelegate). The Kayak type is now wrapped immediately there; nothing downstream sees it:

public void OnRequest(HttpRequestHead request, IDataProducer body, IHttpResponseDelegate response)
{
    IHttpRequestHead requestHead = new KayakHttpRequestHeadAdapter(request);
    // all internal calls use requestHead (IHttpRequestHead)
}

Updated signatures

All public interfaces (IRequestHandler, IRequestVerify, IStubResponse, IMatchingRule, IRequestMatcher) and their implementations now reference IHttpRequestHead instead of Kayak.Http.HttpRequestHead. Unit tests updated accordingly.