Add BeASimulatedFailure assertion (closes #31)

[dennisdoomen]

Summary

Adds a BeASimulatedFailure() assertion to CapturedRequestAssertions, companion to mockly#116.

Changes

• Shared/HttpMockAssertionExtensions.cs: Added BeASimulatedFailure() to CapturedRequestAssertions. Checks subject.SimulatedFailure is not null.
• Specs/AssertionSpecs.cs: Added SimulatedFailureAssertionSpecs nested class with 3 tests (ThrowsException, TimesOut, and failure case).
• **/FluentAssertions.Mockly.*.csproj: Updated Mockly version range to [1.8.0-preview,2.0.0) to include the new SimulatedFailure property.
• API approval files: Regenerated with AcceptApiChanges.ps1.

Notes

⚠️ Draft PR: This depends on Mockly 1.8 which adds SimulatedFailure on CapturedRequest (from mockly#116). The version reference uses a local preview build (1.8.0-preview) and targets [1.8.0-preview,2.0.0). Once Mockly 1.8 is released, this PR should be rebased/updated to use the stable version range.

New API

/// <summary>Asserts that the captured request entry represents a simulated network failure.</summary>
public AndConstraint<CapturedRequestAssertions> BeASimulatedFailure(string because = "", params object[] becauseArgs);

[github-actions]

Test Results

0 files   -   5  0 suites   - 5   0s ⏱️ -13s
0 tests  -  45  0 ✅  -  45  0 💤 ±0  0 ❌ ±0 
0 runs   - 168  0 ✅  - 168  0 💤 ±0  0 ❌ ±0 

Results for commit 9cea1c0. ± Comparison against base commit 88062f7.

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