[API Proposal]: Add response-side assertions (WithResponseStatus, WithResponseBody, WithResponseBodyEquivalentTo)

## Motivation

`WithResponseHeader` is the only assertion in the library that looks at the response. Everything else asserts on the request — even though `CapturedRequest.Response` is right there and holds the full `HttpResponseMessage`.

That means you cannot currently assert "this call returned a 201 with this body" through the Mockly assertions. You have to reach into `capturedRequest.Response` and assert on it by hand.

There is also a signal that this is wanted: the core repo carries a test-only `Mockly.Specs/HttpResponseMessageAssertions.cs` helper. The functionality exists, it just was never shipped to users.

## Proposed API

```csharp
requests.Should().ContainRequestFor("/api/users")
.WithResponseStatus(HttpStatusCode.Created)
.WithResponseBody("*created*")
.WithResponseBodyMatchingJson("{ \"id\": 1 }")
.WithResponseBodyEquivalentTo(new { id = 1, name = "John" });
```

These would sit on `ContainedRequestAssertions` next to the existing `WithResponseHeader`, and mirror the naming of the request-side `WithBody` / `WithBodyMatchingJson` / `WithBodyEquivalentTo` family so there is nothing new to learn.

## Notes

- Reading the response body is async; the existing assertions are all sync. Needs a decision on whether to block on the read or expose async assertions. Since the content is already buffered in memory for a mock, blocking is probably acceptable and keeps the fluent chain intact.
- Consider promoting the core repo's `HttpResponseMessageAssertions` test helper into this package rather than writing it twice.

Start with ContainedRequestAssertions, the existing WithResponseHeader assertion, and the request-side WithBody, WithBodyMatchingJson, and WithBodyEquivalentTo methods. Read Mockly.Specs/HttpResponseMessageAssertions.cs in the core repository and decide how response-body reads should fit the synchronous fluent API. Done means the proposed response status, body, JSON-matching, and equivalent-body assertions are available to users with coverage for their behavior.

Written by the indexing model from the issue text.