feat(gateway): pass through exact upstream token usage in streams

Streaming responses now carry the upstream's real token usage instead of
gateway estimates:
- UnifiedChunk gains an optional Usage field; adapters (opencode, openai)
  extract usage from upstream stream chunks (including the final chunk with
  empty choices) and pass it through.
- standardSSEChunk preserves usage for passthrough adapters.
- Gateway emits the exact usage in the final stream chunk when available,
  falling back to estimates only when the upstream provided none.

Non-streaming usage was already fixed to emit OpenAI-standard keys.

internal/provider/provider.go

@@ -901,20 +901,52 @@ func standardSSEChunk(data string) string {
 			} `json:"delta"`
 			FinishReason *string `json:"finish_reason"`
 		} `json:"choices"`
+		UpstreamUsage 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"`
+		} `json:"usage"`
 	}
 	if err := json.Unmarshal([]byte(data), &raw); err != nil {
 		return ""
 	}
-	if len(raw.Choices) == 0 {
+	if len(raw.Choices) == 0 && raw.UpstreamUsage.Total == 0 && raw.UpstreamUsage.TotalTokens == 0 {
 		return ""
 	}
+	var usage *types.TokenUsage
+	pu := raw.UpstreamUsage
+	if pu.Total > 0 || pu.TotalTokens > 0 {
+		usage = &types.TokenUsage{
+			Prompt:     pickFirst(pu.PromptTokens, pu.Prompt),
+			Completion: pickFirst(pu.CompletionTokens, pu.Completion),
+			Total:      pickFirst(pu.TotalTokens, pu.Total),
+		}
+	}
 	out, _ := json.Marshal(types.UnifiedChunk{
-		Content: raw.Choices[0].Delta.Content,
-		Done:    raw.Choices[0].FinishReason != nil,
+		Content: func() string {
+			if len(raw.Choices) > 0 {
+				return raw.Choices[0].Delta.Content
+			}
+			return ""
+		}(),
+		Done:  len(raw.Choices) > 0 && raw.Choices[0].FinishReason != nil,
+		Usage: usage,
 	})
 	return string(out)
 }
 
+// pickFirst returns a if non-zero, else b (for usage keys that may appear in
+// either standard *_tokens or legacy short form).
+func pickFirst(a, b int) int {
+	if a != 0 {
+		return a
+	}
+	return b
+}
+
 func truncate(s string, n int) string {
 	if len(s) <= n {
 		return s