streaming: only call flush if writes succeeded (#61752)

This is a speculative change to reduce the chances of misuse of the
streaming http event writer (SSE). We are seeing panics in attribution
search since it is trying to write events once the request has finished.

From the stack traces it seems the http package safely handles calls to
Write without panicing but does not do the same for Flush. This change
also seems more correct.

Note: this does not address the root cause of the panics, but should
make it far less likely to occur.

Test Plan: go test

internal/search/streaming/http/writer.go

@@ -106,7 +106,10 @@ func (e *Writer) EventBytes(event string, dataLine []byte) (err error) {
 	write(dataLine)
 	write([]byte("\n\n"))
 
-	e.flush()
+	// only need to call flush if writes succeeded.
+	if err == nil {
+		e.flush()
+	}
 
 	return err
 }