feat(opencode): 透传 completion_tokens_details.reasoning_tokens 与上游 cost

回答「opencodego 的用量与费用透传呢」时逐字段核对上游产出，发现 usage 漏了
一项、费用整项丢失。

## 上游实际发什么（实测 opencode.ai/zen/go/v1）

  {
    "choices": [...],
    "usage": { "prompt_tokens": 37, "completion_tokens": 40, "total_tokens": 77,
               "prompt_cache_hit_tokens": 0, "prompt_cache_miss_tokens": 37,
               "prompt_tokens_details": {"cached_tokens": 0},
               "completion_tokens_details": {"reasoning_tokens": 40} },
    "cost": "0"
  }

cost 在**顶层**且是**字符串**。流式时还会单独发一帧：
{"choices":[],"cost":"0"}

## 此前丢了两样

1. completion_tokens_details.reasoning_tokens —— 输出里有多少是思考 token。
   没有它，客户端无法判断 completion_tokens 里多少是可见回答、多少是思考，
   而两者都按输出计费。
2. cost —— 唯一的费用信号，网关整个丢弃。Go 订阅是包月制恒为 "0"，
   但 Zen 按量付费模型（以及未来的其它源）有信息量。

顺带修掉一处流式/非流式不一致：命中缓存时上游同时给
prompt_tokens_details.cached_tokens 和独立的 hit/miss，流式路径写成了 elseif，
只留 details，与非流式产出不同（只认独立字段的老客户端会看不到缓存）。

## 实现

- types.TokenUsage += CompletionTokensDetails；UnifiedResponse / UnifiedChunk += Cost
- opencodego/opencodezen 适配器映射两个字段；空 choices 帧改成 usage 与 cost
  都可带（早退只带 usage 会把同帧的 cost 丢干净 —— 新测试先抓到的就是这个）
- Gateway ChatCompletion / ChatChunk += cost，随终帧发（对齐上游的
  {"choices":[],"cost":"0"} 形态）
- Go 兜底 standardSSEChunk 同步支持（openai 系适配器不再漏 reasoning_tokens；
  纯 cost 帧不再被整体丢弃），新增 rawCostString 兼容字符串/数字两种形态

费用只做**搬运**：不解析、不换算、不汇总 —— 它是上游事实，且只有部分上游提供。

## 验证

经网关实测 gozen:deepseek-v4.1-flash，流式与非流式产出逐字段一致：
  prompt_tokens_details.cached_tokens=6784
  prompt_cache_hit_tokens=6784 / miss=148
  completion_tokens_details.reasoning_tokens=16
  cost="0"

测试：TestOpenCodeCostAndReasoningPassthrough（含「无数据不得凭空造字段」反例）、
TestOpenCodeStreamCacheFieldsMatchNonStream、TestTokenUsageMarshalsCompletionTokensDetails。

internal/gateway/chat.go

@@ -42,6 +42,10 @@ type ChatCompletion struct {
 	Model   string            `json:"model"`
 	Choices []ChatChoice      `json:"choices"`
 	Usage   *types.TokenUsage `json:"usage,omitempty"`
+	// Cost is the upstream-reported charge for this request, passed through
+	// verbatim (OpenCode reports it as a decimal string, always "0" on the
+	// flat-rate Go subscription). Absent when the upstream reports nothing.
+	Cost string `json:"cost,omitempty"`
 }
 
 type ChatChoice struct {
@@ -66,6 +70,10 @@ type ChatChunk struct {
 	// Usage is sent in the final chunk of a stream (empty choices) so
 	// OpenAI-compatible clients can read token usage.
 	Usage *types.TokenUsage `json:"usage,omitempty"`
+	// Cost is the upstream-reported charge for this request, passed through
+	// verbatim (OpenCode reports it as a decimal string). Emitted on the
+	// terminal chunk, mirroring OpenCode's own {"choices":[],"cost":"0"}.
+	Cost string `json:"cost,omitempty"`
 }
 
 type ChunkChoice struct {
@@ -703,6 +711,7 @@ func writeChatCompletion(w http.ResponseWriter, resp *types.UnifiedResponse, mod
 		Created: time.Now().Unix(),
 		Model:   modelName,
 		Choices: []ChatChoice{{Index: 0, Message: msg, FinishReason: resp.FinishReason}},
+		Cost:    resp.Cost,
 	}
 	if resp.TokenUsage.Total > 0 || resp.TokenUsage.Prompt > 0 || resp.TokenUsage.Completion > 0 {
 		out.Usage = &resp.TokenUsage
@@ -822,11 +831,15 @@ func (g *Gateway) pumpStream(w http.ResponseWriter, rec *Req, chunks <-chan type
 	// source's status-page latency average.
 	rec.FirstByteMs = time.Since(t0).Milliseconds()
 	var lastUsage *types.TokenUsage
+	var lastCost string
 	lastFinish := ""
 	for ck := range chunks {
 		if ck.Usage != nil {
 			lastUsage = mergeUsage(lastUsage, ck.Usage)
 		}
+		if ck.Cost != "" {
+			lastCost = ck.Cost
+		}
 		chunk := ChatChunk{
 			ID: id, Object: "chat.completion.chunk", Created: created, Model: modelName,
 		}
@@ -893,6 +906,14 @@ func (g *Gateway) pumpStream(w http.ResponseWriter, rec *Req, chunks <-chan type
 			ID: id, Object: "chat.completion.chunk", Created: created, Model: modelName,
 			Choices: []ChunkChoice{},
 			Usage:   tut,
+			Cost:    lastCost,
+		})
+	} else if lastCost != "" {
+		// Cost arrived without any usage (OpenCode sends it on its own frame).
+		send(ChatChunk{
+			ID: id, Object: "chat.completion.chunk", Created: created, Model: modelName,
+			Choices: []ChunkChoice{},
+			Cost:    lastCost,
 		})
 	}
 	fmt.Fprintf(w, "data: [DONE]\n\n")