fix(api): Don't send responses twice.
In some cases multiple responses are being sent for one request, causing
the API server to incorrectly drop connections.
This change introduces a ResponseWriter which tracks whether a
response has already been sent. This is used to prevent a response being
sent if something already has (e.g. by a preconditions check function).
Fixes#21633.
Co-authored-by: Menno Finlay-Smits <hello@menno.io>
`go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...` executed.
`go generate ./...` ran afterwards to keep generated.
