Rename type name ResponseHandlerDelegate so that it does not end in 'Delegate' (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1711)

Properties should not return arrays (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1819)

Exception type System.IndexOutOfRangeException is reserved by the runtime (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca2201)

In virtual/interface member IResponseBehavior.HandleAsync(MockHttpRequestContext, HttpResponseMessage, ResponseHandlerDelegate, CancellationToken), rename parameter next so that it no longer conflicts with the reserved language keyword 'Next'. Using a reserved keyword as the name of a parameter on a virtual/interface member makes it harder for consumers in other languages to override/implement the member. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1716)

Consider calling ConfigureAwait on the awaited task (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca2007)

Modify 'TryAdd' to catch a more specific allowed exception type, or rethrow the exception (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1031)

Change type of field 'HeadersWithSingleValueOnly' from 'System.Collections.Generic.ISet<string>' to 'System.Collections.Generic.HashSet<string>' for improved performance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1859)

Prefer comparing 'Count' to 0 rather than using 'Any()', both for clarity and for performance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1860)

Random is an insecure random number generator. Use cryptographically secure random number generators when randomness is required for security. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca5394)