Add BeASimulatedFailure assertion (companion to mockly#116)

[dennisdoomen]

Companion to dennisdoomen/mockly#116 (simulated network failures: ThrowsException / TimesOut), PR dennisdoomen/mockly#136.

Summary

Add an assertion to verify that a captured entry corresponds to an intended simulated failure (a thrown transport error / timeout) rather than an unexpected 500 or a normal response.

Proposed API (additive)

On CapturedRequestAssertions in Shared/HttpMockAssertionExtensions.cs (next to the existing BeExpected() / BeUnexpected()):

AndConstraint<CapturedRequestAssertions> BeASimulatedFailure(string because = "", params object[] becauseArgs);

Implementation notes

• Follow the existing BeExpected/BeUnexpected patterns.
• Update BOTH v7 and v8 packages (shared via Shared/); add specs to both .Specs projects.
• Run AcceptApiChanges.ps1 and commit updated approved-API files.

Dependency / blocker (IMPORTANT)

This needs the core Mockly to expose a public signal on CapturedRequest indicating the entry was a simulated failure (e.g. a SimulatedException / WasSimulatedFailure property). The core PR dennisdoomen/mockly#136 records the failed call in Requests but may not yet expose such a flag — confirm, and if missing, file/extend a small core change first. Implement after dennisdoomen/mockly#116 (PR #136) is merged and a new Mockly package is published.