[API Proposal]: Add ordering, count and timing assertions on RequestCollection

## Motivation

Ordering can only be asserted through `HttpMockAssertions.HaveCalledInOrder(params RequestMockResponseBuilder[])`. That works, but it requires you to keep a reference to every builder:

```csharp
var auth = mock.ForPost("/token").RespondsWithStatus(HttpStatusCode.OK);
var data = mock.ForGet("/api/data").RespondsWithJsonContent(payload);
// ...
mock.Should().HaveCalledInOrder(auth, data);
```

If you are asserting on a captured `RequestCollection` rather than the mock itself, there is no ordering assertion at all — even though `CapturedRequest` already exposes both `Sequence` and `Timestamp`.

## Proposed API

Ordering by URL pattern, no builder references needed:

```csharp
requests.Should().ContainRequestsInOrder("/token", "/api/data");
requests.Should().ContainRequestsInOrder(
(HttpMethod.Post, "/token"),
(HttpMethod.Get, "/api/data"));
```

Count assertions, which are also missing today:

```csharp
requests.Should().ContainRequestsFor("/api/users", Exactly.Times(3));
```

Timing, useful together with the core `After(TimeSpan)` latency feature:

```csharp
requests.Should().AllHaveBeenSentWithin(TimeSpan.FromSeconds(5));
```

## Notes

- `Sequence` already drives `HaveCalledInOrder`, so the ordering logic can largely be reused.
- Failure messages should print the actual observed order, which `HaveCalledInOrder` already does well.
- The timing assertion is the least certain of the three — worth confirming it does not just encourage flaky tests.

Start by reading HttpMockAssertions.HaveCalledInOrder and the RequestCollection and CapturedRequest APIs, especially Sequence and Timestamp. Compare the proposed ordering, count, and timing assertions with the existing behavior, and confirm whether timing avoids encouraging flaky tests. Done means the agreed RequestCollection assertions and their observed-order or timing failure behavior are covered.

Written by the indexing model from the issue text.