Random error plugins: rate 99 behaves like 100 and all rates are slightly off #1926

Description

GenericRandomErrorPlugin and GraphRandomErrorPlugin don't apply the configured failure rate exactly. Both use:

_random.Next(1, 100) <= Configuration.Rate

Random.Next(1, 100) returns values from 1 to 99 (upper bound is exclusive), so:

• rate: 99 behaves like rate: 100 (every request fails)
• every other rate is slightly higher than configured (for example, rate: 50 fails ~50.5% of requests, rate: 1 ~1.01%)

Code:

• dev-proxy/DevProxy.Plugins/Behavior/GenericRandomErrorPlugin.cs

  Line 142 in d0965ec

  private GenericRandomErrorFailMode ShouldFail() => _random.Next(1, 100) <= Configuration.Rate ? GenericRandomErrorFailMode.Random : GenericRandomErrorFailMode.PassThru;

• dev-proxy/DevProxy.Plugins/Behavior/GraphRandomErrorPlugin.cs

  Line 210 in d0965ec

  private GraphRandomErrorFailMode ShouldFail() => _random.Next(1, 100) <= Configuration.Rate ? GraphRandomErrorFailMode.Random : GraphRandomErrorFailMode.PassThru;

Expected behavior

rate: N fails exactly N% of requests on average, for example by using _random.Next(0, 100) < Configuration.Rate.

Steps to reproduce

1. Configure GenericRandomErrorPlugin with "rate": 99
2. Send many requests through the proxy
3. Observe that every request fails