fix: stream return EOF when openai return error (#184)

* fix: stream return EOF when openai return error

* perf: add error accumulator

* fix: golangci-lint

* fix: unmarshal error possibly null

* fix: error accumulator

* test: error accumulator use interface and add test code

* test: error accumulator add test code

* refactor: use stream reader to re-use stream code

* refactor: stream reader use generics

unmarshaler.go

@@ -0,0 +1,15 @@
+package openai
+
+import (
+	"encoding/json"
+)
+
+type unmarshaler interface {
+	unmarshal(data []byte, v any) error
+}
+
+type jsonUnmarshaler struct{}
+
+func (jm *jsonUnmarshaler) unmarshal(data []byte, v any) error {
+	return json.Unmarshal(data, v)
+}