frankenphp

afb5e844 - fix: don't warn on routine client-disconnect write/flush errors (#2589)

fix: don't warn on routine client-disconnect write/flush errors (#2589) ## Summary While investigating https://github.com/php/frankenphp/issues/2588, found two related logging bugs around client disconnects. `go_sapi_flush` logged every `ResponseController.Flush()` error under the same message: > the current responseWriter is not a flusher, if you are not using a custom build, please report this issue That message is only accurate for `http.ErrNotSupported`. A plain disconnect/write error (e.g. an aborted HTTP/2 stream) got mislabeled the same way, misleading and able to mask the real cause when triaging issues shaped like #2588. Also reconsidered the log level. `go_ub_write`'s equivalent "write error" was logged at warn, so the new "flush error" branch was first matched to it. But a client disconnecting mid-response is routine, and both writes and flushes happen in a loop, so a single request can log this dozens or hundreds of times, one per write/flush call after the disconnect. Not warn-worthy. Checked how the two other Caddy-family projects here handle this same class of error: - Caddy's reverse proxy streaming (`streaming.go`): "backConn write failed", "response flush", "streaming error" - all logged at debug. - Mercure's SSE hub (`subscribe.go`): "Failed to write comment" (per-write, in a loop) and "Connection closed by the client" are debug; only one-off lifecycle events like "Subscriber disconnected" are info. Matched that convention: - `go_ub_write`'s "write error" -> debug (was warn). - `go_sapi_flush`'s "flush error" -> debug (routine disconnect), correctly separated from "not a flusher". - `go_sapi_flush`'s "not a flusher" -> stays warn (real misconfiguration, not a per-request occurrence). This only fixes logging, not the segfault. I was not able to reproduce the crash reported in #2588 (see comment on the issue). ## Test plan - `go build ./...` - `go vet ./...` - `go test -run "TestConnectionAbort|TestLog" ./...`