[Code Review] The test asserts only IsType<ArgumentNullException> without checking ParamName. If the guard in BotService is accidentally changed to throw new ArgumentNullException(nameof(botMessageChannel)) (wrong parameter name), the test still passes and the regression goes undetected.

Suggestion: also assert the parameter name:

ArgumentNullException ane = Assert.IsType<ArgumentNullException>(ex.InnerException);
Assert.Equal("bot", ane.ParamName);

[Code Review] Assert.Single(typeof(BotService).GetConstructors()) is a guard against a second constructor being added, but if one is ever added the failure message will be a count-related xunit error with no mention of null guards — making the root cause non-obvious to the next developer.

Consider selecting the specific constructor by its parameter types instead, which both locates the right overload and provides a clearer failure if the signature changes:

ConstructorInfo? ctor = typeof(BotService).GetConstructor([
    typeof(IDiscordBot),
    typeof(IMessageChannel<BotMessage>),
    typeof(IMessageChannel<BotReleaseMessage>)
]);
Assert.NotNull(ctor);

[Code Review] messageChannel and releaseChannel are created but never reached: the BotService constructor throws ArgumentNullException on the first line (bot ?? throw ...) before it touches either channel parameter. Passing null for those positions (or any placeholder) would produce the same result and make it clearer that only bot is the subject under test.