Prevent malformed Responses SSE frames from breaking stream clients

Line-oriented upstream executors can emit `event:` and `data:` as
separate chunks, but the Responses handler had started terminating
each incoming chunk as a full SSE event. That split `response.created`
into an empty event plus a later data block, which broke downstream
clients like OpenClaw.

This keeps the fix in the handler layer: a small stateful framer now
buffers standalone `event:` lines until the matching `data:` arrives,
preserves already-framed events, and ignores delimiter-only leftovers.
The regression suite now covers split event/data framing, full-event
passthrough, terminal errors, and the bootstrap path that forwards
line-oriented openai-response streams from non-Codex executors too.

Constraint: Keep the fix localized to Responses handler framing instead of patching every executor
Rejected: Revert to v6.9.7 chunk writing | would reintroduce data-only framing regressions
Rejected: Patch each line-oriented executor separately | duplicates fragile SSE assembly logic
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Do not assume incoming Responses stream chunks are already complete SSE events; preserve handler-layer reassembly for split `event:`/`data:` inputs
Tested: /tmp/go1.26.1/go/bin/go test ./sdk/api/handlers/openai -count=1
Tested: /tmp/go1.26.1/go/bin/go test ./sdk/api/handlers -count=1
Tested: /tmp/go1.26.1/go test ./sdk/api/handlers/... -count=1
Tested: /tmp/go1.26.1/go/bin/go vet ./sdk/api/handlers/...
Tested: Temporary patched server on 127.0.0.1:18317 -> /v1/models 200, /v1/responses non-stream 200, /v1/responses stream emitted combined `event:` + `data:` frames
Not-tested: Full repository test suite outside sdk/api/handlers packages

sdk/api/handlers/openai/openai_responses_handlers_stream_test.go

@@ -12,7 +12,9 @@ import (
 	sdkconfig "github.com/router-for-me/CLIProxyAPI/v6/sdk/config"
 )
 
-func TestForwardResponsesStreamSeparatesDataOnlySSEChunks(t *testing.T) {
+func newResponsesStreamTestHandler(t *testing.T) (*OpenAIResponsesAPIHandler, *httptest.ResponseRecorder, *gin.Context, http.Flusher) {
+	t.Helper()
+
 	gin.SetMode(gin.TestMode)
 	base := handlers.NewBaseAPIHandlers(&sdkconfig.SDKConfig{}, nil)
 	h := NewOpenAIResponsesAPIHandler(base)
@@ -26,6 +28,12 @@ func TestForwardResponsesStreamSeparatesDataOnlySSEChunks(t *testing.T) {
 		t.Fatalf("expected gin writer to implement http.Flusher")
 	}
 
+	return h, recorder, c, flusher
+}
+
+func TestForwardResponsesStreamSeparatesDataOnlySSEChunks(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+
 	data := make(chan []byte, 2)
 	errs := make(chan *interfaces.ErrorMessage)
 	data <- []byte("data: {\"type\":\"response.output_item.done\",\"item\":{\"type\":\"function_call\",\"arguments\":\"{}\"}}")
@@ -33,7 +41,7 @@ func TestForwardResponsesStreamSeparatesDataOnlySSEChunks(t *testing.T) {
 	close(data)
 	close(errs)
 
-	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs)
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, nil)
 	body := recorder.Body.String()
 	parts := strings.Split(strings.TrimSpace(body), "\n\n")
 	if len(parts) != 2 {
@@ -50,3 +58,41 @@ func TestForwardResponsesStreamSeparatesDataOnlySSEChunks(t *testing.T) {
 		t.Errorf("unexpected second event.\nGot: %q\nWant: %q", parts[1], expectedPart2)
 	}
 }
+
+func TestForwardResponsesStreamReassemblesSplitSSEEventChunks(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+
+	data := make(chan []byte, 3)
+	errs := make(chan *interfaces.ErrorMessage)
+	data <- []byte("event: response.created")
+	data <- []byte("data: {\"type\":\"response.created\",\"response\":{\"id\":\"resp-1\"}}")
+	data <- []byte("\n")
+	close(data)
+	close(errs)
+
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, nil)
+
+	got := strings.TrimSuffix(recorder.Body.String(), "\n")
+	want := "event: response.created\ndata: {\"type\":\"response.created\",\"response\":{\"id\":\"resp-1\"}}\n\n"
+	if got != want {
+		t.Fatalf("unexpected split-event framing.\nGot:  %q\nWant: %q", got, want)
+	}
+}
+
+func TestForwardResponsesStreamPreservesValidFullSSEEventChunks(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+
+	data := make(chan []byte, 1)
+	errs := make(chan *interfaces.ErrorMessage)
+	chunk := []byte("event: response.created\ndata: {\"type\":\"response.created\",\"response\":{\"id\":\"resp-1\"}}\n\n")
+	data <- chunk
+	close(data)
+	close(errs)
+
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, nil)
+
+	got := strings.TrimSuffix(recorder.Body.String(), "\n")
+	if got != string(chunk) {
+		t.Fatalf("unexpected full-event framing.\nGot:  %q\nWant: %q", got, string(chunk))
+	}
+}