fix(gateway): emit OpenAI-standard token usage in responses

- TokenUsage.MarshalJSON now emits both standard (prompt_tokens,
  completion_tokens, total_tokens) and legacy (prompt, completion, total)
  keys, so OpenAI-compatible clients (DSH, DevEco Code, etc.) can read
  token usage.
- ChatChunk gains an optional Usage field; stream responses now send a
  final usage chunk (empty choices) before [DONE].

Refs: usage not visible in clients because the gateway serialized only the
internal short keys and never emitted a streaming usage chunk.

internal/types/types.go

@@ -85,6 +85,28 @@ type TokenUsage struct {
 	Total      int `json:"total"`
 }
 
+// MarshalJSON emits both the legacy short keys (prompt/completion/total, used
+// by the internal unified representation and older clients) and the OpenAI
+// standard keys (prompt_tokens/completion_tokens/total_tokens). Standard
+// clients such as DSH and DevEco Code read the *_tokens fields.
+func (t TokenUsage) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		PromptTokens     int `json:"prompt_tokens"`
+		CompletionTokens int `json:"completion_tokens"`
+		TotalTokens      int `json:"total_tokens"`
+		Prompt           int `json:"prompt"`
+		Completion       int `json:"completion"`
+		Total            int `json:"total"`
+	}{
+		PromptTokens:     t.Prompt,
+		CompletionTokens: t.Completion,
+		TotalTokens:      t.Total,
+		Prompt:           t.Prompt,
+		Completion:       t.Completion,
+		Total:            t.Total,
+	})
+}
+
 type ImageData struct {
 	B64JSON string `json:"b64_json,omitempty"`
 	URL     string `json:"url,omitempty"`