Race condition in cancelation can result in "unknown" error code instead of "canceled"

[jhump]

Here's a conformance failure that demonstrates the issue:
https://github.com/connectrpc/connect-kotlin/actions/runs/10888115561/job/30211872724

[jhump]

The issue seems to be that okhttp (or this repo's okhttp adapter) sometimes treats the closure of the HTTP 1.1 connection as a plain EOF (resulting in an unknown error code) and sometimes as a cancelation (which results in the correct code).

It may depend on timing, such as suggested in this issue title, or perhaps there is something slightly different on the wire that distinguishes the two (like related to chunk encoding of the response and the boundary where the cancellation occurs?).