rpc: fix rare deadlock when canceling HTTP call context (#19715)

When cancelling the context for a call on a HTTP-based client while the
call is running, the select in requestOp.wait may hit the <-context.Done()
case instead of the <-op.resp case. This doesn't happen often -- our
cancel test hasn't caught this even though it ran thousands of times
on CI since the RPC client was added.

Fixes #19714