WebTransport PR follow-ups

To avoid making the WebTransport PR even more massive and because of time constraints, I was not able to implement everything in it. This issue outlines some smaller follow up tasks that were identified during the PR's development and review. The larger items will receive their own issues. I have sorted the lists based on what, I think, is the importance/usefulness to the customers.

- [ ] Poison the message body and response body so users don't go using them
- [ ] Discard cancelled streams before users can accept them
- [ ] Add to docs and blog post
- [ ] Making webtransportsession a multiplexeconnectioncontext - not sure if it makes sense or not as there is a rather small overlap in functionality
- [ ] Server-initiated bidirectional stream partial closing - currently there is no way to close a single side of the bidirectional stream from the server. The JS client API does have a system for this. So, it might make sense to add it to the server-side as well.
- [ ] Limit and make sure we cleanup [`_webtransportSessions`](https://github.com/dotnet/aspnetcore/blob/7b520eb5d33f41cbea49ff88e360b8cf08b5aa94/src/Servers/Kestrel/Core/src/Internal/Http3/Http3Connection.cs#L43) and [`_unidentifiedStreams`](https://github.com/dotnet/aspnetcore/blob/7b520eb5d33f41cbea49ff88e360b8cf08b5aa94/src/Servers/Kestrel/Core/src/Internal/Http3/Http3Connection.cs#L26)

## Big features that have their own issues
- [ ] https://github.com/dotnet/aspnetcore/issues/42789
- [ ] https://github.com/dotnet/aspnetcore/issues/42784
- [ ] https://github.com/dotnet/aspnetcore/issues/42786
- [x] https://github.com/dotnet/aspnetcore/issues/42787
- [ ] https://github.com/dotnet/aspnetcore/issues/42785

Choose one unchecked follow-up before starting, then read the referenced Http3Connection.cs locations for _webtransportSessions and _unidentifiedStreams and review the related issues 42789, 42784, 42786, and 42785. The issue contains several independent tasks, some explicitly uncertain, so completion criteria must be established for the selected task.

Written by the indexing model from the issue text.