Remove TODO

[lukebakken]

Add assertion

[lukebakken]

@stebet I hope you have some time to comment about this. When reviewing the pipelines code you added to this library (thank you again!) I noticed that the IsSingleSegment property of the PipeReader's buffer was not checked and thus the code assumes that the buffer is a single segment. I only thought to check this due to reading @mgravell's blog posts about `System.IO.Pipelines.

So, in this PR, I added an assertion that the buffer is a single segment:

https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/lukebakken/todos/projects/RabbitMQ.Client/client/impl/Frame.cs#L317

...and, sure enough, the assertion does not always hold:

https://github.com/rabbitmq/rabbitmq-dotnet-client/actions/runs/8241012258/job/22537494233

What's interesting is that running that test in isolation, on my local machine, does not reproduce the assertion failure. Only running the entire suite does.

I'm going to see if I can handle this case efficiently, but I would love a second opinion.

cc-ing @bording and @danielmarbach in case they have some input.

UPDATE: I think I answered my own question:
b680b90

[bording]

@lukebakken A quick look at the section of code in question, I think it's fine because there isn't an optimized path that's doing something different when there is a single segment (no place that assumes buffer.First is the entire set of data).

[stebet]

Good catch @lukebakken!

Edit: Just saw the explanation :)