[API Proposal]: Make HttpMock disposable with an opt-in Strict mode (README's `using var mock` does not compile)

## Bug

The README's headline sample and the "Power in Simplicity" section both start with:

```csharp
using var mock = new HttpMock();
```

`HttpMock` does not implement `IDisposable` (see `Mockly.ApiVerificationTests/ApprovedApi/net8.0.verified.txt`), so that snippet does not compile. Anyone copying the first sample off the README hits a compiler error immediately.

## Proposal

Rather than just dropping `using` from the docs, make `HttpMock` disposable and give disposal a purpose:

```csharp
using var mock = new HttpMock { Strict = true };
// ... test body ...
// on Dispose: throws if any configured mock was never invoked,
// or if unexpected calls were recorded
```

`Strict` would default to `false`, so nothing changes for existing users. With it on, you no longer have to remember `HaveAllRequestsCalled()` at the end of every test, and a test that forgets its assertions still fails.

## Notes

- Dispose must not throw while an exception is already in flight, or it will mask the real failure. Needs care around `Marshal.GetExceptionPointers()` / a "did the test already fail" heuristic, or a documented caveat.
- Also lets `HttpMock` dispose the `HttpClient` instances it hands out.
- Either way, the README needs fixing — if `Strict` is rejected, drop `using` from the samples.

Start with the README samples and Mockly.ApiVerificationTests/ApprovedApi/net8.0.verified.txt, then locate HttpMock and its existing request-verification behavior. Decide whether the disposable Strict proposal is feasible, including exception-in-flight handling and HttpClient disposal; done means the API verification remains correct, strict disposal reports unmet expectations without masking test failures, and the README sample compiles.

Written by the indexing model from the issue text.