feat: record stream interruption reasons via StreamStatus

- Add StreamStatus type (relay/common) to track stream end reason
  (done/timeout/client_gone/scanner_error/eof/panic/ping_fail) and
  accumulate soft errors during streaming via sync.Once + sync.Mutex.
- Add StreamResult (relay/helper) as the callback interface: adapters
  call sr.Error() for soft errors, sr.Stop() for fatal, sr.Done() for
  normal completion. No early-return problem — multiple errors per chunk
  are naturally supported.
- Refactor StreamScannerHandler callback from func(string) bool to
  func(string, *StreamResult). All 9 channel adapters updated.
- Write stream_status into log other JSON field (admin-only) with
  status ok/error, end_reason, error_count, and error messages.
- Frontend: display stream status in log detail expansion for admins.

relay/helper/stream_result.go

@@ -0,0 +1,52 @@
+package helper
+
+import (
+	relaycommon "github.com/QuantumNous/new-api/relay/common"
+)
+
+// StreamResult is passed to each dataHandler invocation, providing methods
+// to record soft errors, signal fatal stops, or mark normal completion.
+// StreamScannerHandler checks IsStopped() after each callback invocation.
+type StreamResult struct {
+	status  *relaycommon.StreamStatus
+	stopped bool
+}
+
+func newStreamResult(status *relaycommon.StreamStatus) *StreamResult {
+	return &StreamResult{status: status}
+}
+
+// Error records a soft error. The stream continues processing.
+// Can be called multiple times per chunk.
+func (r *StreamResult) Error(err error) {
+	if err == nil {
+		return
+	}
+	r.status.RecordError(err.Error())
+}
+
+// Stop records a fatal error and marks the stream to stop after this chunk.
+func (r *StreamResult) Stop(err error) {
+	if err != nil {
+		r.status.RecordError(err.Error())
+	}
+	r.status.SetEndReason(relaycommon.StreamEndReasonHandlerStop, err)
+	r.stopped = true
+}
+
+// Done signals that the handler has finished processing normally
+// (e.g., Dify "message_end"). The stream stops after this chunk.
+func (r *StreamResult) Done() {
+	r.status.SetEndReason(relaycommon.StreamEndReasonDone, nil)
+	r.stopped = true
+}
+
+// IsStopped returns whether Stop() or Done() was called during this chunk.
+func (r *StreamResult) IsStopped() bool {
+	return r.stopped
+}
+
+// reset clears the per-chunk stopped flag so the object can be reused.
+func (r *StreamResult) reset() {
+	r.stopped = false
+}