fix: properly terminate inbound SSE connections

[cwaldren-ld]

There are various contract tests that do something like:

serveStreamingConnection()
assertSomething()

breakConnection()

serverAnotherStreamingConnection()
assertSomethingElse()

It appears that the code that was supposed to terminate inbound HTTP connections (breakConnection) was incorrect/not doing anything.

Reading the docs for http.Request:

// Context returns the request's context. To change the context, use
// WithContext.
//
// The returned context is always non-nil; it defaults to the
// background context.
//
// For outgoing client requests, the context controls cancellation.
//
// For incoming server requests, the context is canceled when the
// client's connection closes, the request is canceled (with HTTP/2),
// or when the ServeHTTP method returns.

Note the emphasis. For incoming server requests, the context is canceled by the server, not by calling the context.Cancel function. Therefore, all the machinery related to storing context cancellation functions, invoking them, etc. is not going to work - we should let the handlers return, and the server do its job.

I've also added a new contract test to be sure that clients which don't support restart capability still have their re-connection code exercised.

[keelerm84]

No one has been added as a reviewer, but the changes seem sane based on what you say.