Add simulated network failures (ThrowsException / TimesOut) for resilience testing

[dennisdoomen]

Summary

Allow a mock to throw/simulate a network-level failure instead of returning a response, so tests can verify retry, circuit-breaker, and error-handling code.

Motivation

Resilient HTTP code must handle transport failures (HttpRequestException), timeouts (TaskCanceledException), and socket errors. There is no first-class way to simulate these today — and note that the current pipeline actually swallows responder exceptions: RequestMock.TrackRequest catches any exception and converts it into a 500 response, so a user cannot simulate a thrown transport error even via RespondsWith(_ => throw ...).

Proposed API (additive)

RequestMockResponseBuilder ThrowsException(Exception exception);
RequestMockResponseBuilder ThrowsException<TException>() where TException : Exception, new();
RequestMockResponseBuilder TimesOut(); // throws TaskCanceledException, mimicking HttpClient timeout

Usage:

mock.ForGet().WithPath("/flaky").ThrowsException<HttpRequestException>();

Implementation plan

dennisdoomen/mockly (core)

• Add an async response path (see shared dependency below) where an "exception responder" is awaited and the exception is propagated to the caller rather than caught.
• Carefully scope the existing try/catch in TrackRequest (lines ~268–280): distinguish "intentionally simulated transport failure" (propagate) from "bug in a user responder" (current 500 behavior). Represent simulated failures as a dedicated responder type that the pipeline rethrows before/around the catch.
• Ensure the thrown request is still recorded in HttpMock.Requests (as expected/handled) before throwing.
• Tests: HttpRequestException surfaces to HttpClient caller; TimesOut() throws TaskCanceledException; the failed call is captured in Requests; Once()/Times(n) still apply.

dennisdoomen/fluentassertions.mockly (assertions — companion work)

• CapturedRequestAssertions currently has BeExpected() / BeUnexpected(). Add:

  BeASimulatedFailure(string because = "")   // asserts CapturedRequest was a simulated throw, not a real response

  This lets tests verify that the captured entry corresponds to an intended simulated failure rather than an unexpected 500.
• Both v7/v8 packages must be updated via Shared/.
• Needs AcceptApiChanges.ps1 in the FA.Mockly repo.

Shared dependency

Shares the async pipeline + CancellationToken work with the latency (#3) and async-responder (#7) issues. Recommend implementing that plumbing once.

Breaking-change analysis

• None for consumers at the API level (additive methods).
• Behavioral nuance: scoping the existing catch-all so simulated exceptions propagate is a deliberate change. Existing code relying on a thrown responder producing a 500 is unaffected (opt-in via new methods).
• Core: api-approved workflow + AcceptApiChanges.ps1 in dennisdoomen/mockly.
• FA.Mockly: AcceptApiChanges.ps1 in dennisdoomen/fluentassertions.mockly.