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/provider/provider.go

@@ -1518,12 +1518,24 @@ func standardSSEChunk(data string) string {
 			PromptTokensDetails *struct {
 				CachedTokens int `json:"cached_tokens"`
 			} `json:"prompt_tokens_details"`
+			CompletionTokensDetails *struct {
+				ReasoningTokens int `json:"reasoning_tokens"`
+			} `json:"completion_tokens_details"`
 		} `json:"usage"`
+		// Cost is a top-level decimal string on OpenCode Zen/Go responses.
+		Cost json.RawMessage `json:"cost"`
 	}
 	if err := json.Unmarshal([]byte(data), &raw); err != nil {
 		return ""
 	}
 	if len(raw.Choices) == 0 && raw.UpstreamUsage.Total == 0 && raw.UpstreamUsage.TotalTokens == 0 {
+		// Cost-only chunk: OpenCode sends the charge on its own frame
+		// {"choices":[],"cost":"0"} with no usage at all. Dropping it here
+		// would lose the only cost signal the client can see.
+		if c := rawCostString(raw.Cost); c != "" {
+			out, _ := json.Marshal(types.UnifiedChunk{Cost: c})
+			return string(out)
+		}
 		return ""
 	}
 	var usage *types.TokenUsage
@@ -1541,6 +1553,11 @@ func standardSSEChunk(data string) string {
 				CachedTokens: pu.PromptTokensDetails.CachedTokens,
 			}
 		}
+		if pu.CompletionTokensDetails != nil {
+			usage.CompletionTokensDetails = &types.CompletionTokensDetails{
+				ReasoningTokens: pu.CompletionTokensDetails.ReasoningTokens,
+			}
+		}
 	}
 	finish := ""
 	done := false
@@ -1565,6 +1582,24 @@ func standardSSEChunk(data string) string {
 	return string(out)
 }
 
+// rawCostString normalizes an upstream cost field into a plain string. The
+// value is a decimal **string** on OpenCode Zen/Go ("0", "0.0012"), but other
+// upstreams have been seen to send a bare number, so both are accepted.
+// Returns "" when absent, null, or empty.
+func rawCostString(raw json.RawMessage) string {
+	s := strings.TrimSpace(string(raw))
+	if s == "" || s == "null" {
+		return ""
+	}
+	if s[0] == '"' {
+		var unq string
+		if json.Unmarshal(raw, &unq) == nil {
+			return strings.TrimSpace(unq)
+		}
+	}
+	return s
+}
+
 // 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 {