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/types/types.go

@@ -117,6 +117,11 @@ type UnifiedResponse struct {
 	ToolCalls        []ToolCall `json:"tool_calls,omitempty"`
 	// ImageData used by image-generation adapters.
 	ImageData []ImageData `json:"image_data,omitempty"`
+	// Cost is the upstream-reported charge for this request, passed through
+	// verbatim (OpenCode Zen/Go put a decimal **string** at the response top
+	// level). Deliberately not parsed or summed by the gateway: it is an
+	// upstream fact, and only some upstreams report it at all.
+	Cost string `json:"cost,omitempty"`
 }
 
 type TokenUsage struct {
@@ -133,6 +138,16 @@ type TokenUsage struct {
 	// prompt_tokens_details.cached_tokens is absent.
 	PromptCacheHit  int `json:"prompt_cache_hit_tokens,omitempty"`
 	PromptCacheMiss int `json:"prompt_cache_miss_tokens,omitempty"`
+	// CompletionTokensDetails mirrors the OpenAI v2 usage.completion_tokens_details
+	// object. OpenCode Zen/Go report how much of the completion was thinking
+	// tokens here; without it clients cannot tell how much of the billed output
+	// was reasoning rather than visible answer.
+	CompletionTokensDetails *CompletionTokensDetails `json:"completion_tokens_details,omitempty"`
+}
+
+// CompletionTokensDetails is the OpenAI v2 completion_tokens_details object.
+type CompletionTokensDetails struct {
+	ReasoningTokens int `json:"reasoning_tokens"`
 }
 
 // PromptTokensDetails is the OpenAI v2 prompt_tokens_details object. Only
@@ -157,15 +172,16 @@ func (t TokenUsage) MarshalJSON() ([]byte, error) {
 		pdetails = &PromptTokensDetails{CachedTokens: t.PromptCacheHit}
 	}
 	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"`
-		PromptTokensDetails   *PromptTokensDetails `json:"prompt_tokens_details,omitempty"`
-		PromptCacheHitTokens  int                  `json:"prompt_cache_hit_tokens,omitempty"`
-		PromptCacheMissTokens int                  `json:"prompt_cache_miss_tokens,omitempty"`
+		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"`
+		PromptTokensDetails   *PromptTokensDetails     `json:"prompt_tokens_details,omitempty"`
+		PromptCacheHitTokens  int                      `json:"prompt_cache_hit_tokens,omitempty"`
+		PromptCacheMissTokens int                      `json:"prompt_cache_miss_tokens,omitempty"`
+		CompletionDetails     *CompletionTokensDetails `json:"completion_tokens_details,omitempty"`
 	}{
 		PromptTokens:          t.Prompt,
 		CompletionTokens:      t.Completion,
@@ -176,6 +192,7 @@ func (t TokenUsage) MarshalJSON() ([]byte, error) {
 		PromptTokensDetails:   pdetails,
 		PromptCacheHitTokens:  t.PromptCacheHit,
 		PromptCacheMissTokens: t.PromptCacheMiss,
+		CompletionDetails:     t.CompletionTokensDetails,
 	})
 }
 
@@ -219,6 +236,9 @@ type UnifiedChunk struct {
 	// often has empty choices). Gateway uses it to emit exact usage in the
 	// final stream chunk instead of estimates.
 	Usage *TokenUsage `json:"usage,omitempty"`
+	// Cost is upstream-reported charge for this request (decimal string),
+	// passed through verbatim; OpenCode sends it on its own stream chunk.
+	Cost string `json:"cost,omitempty"`
 }
 
 // Meta passed to Lua build_headers hook

internal/types/types_test.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 )
 
@@ -117,3 +118,37 @@ func TestChatMessageNormalizesEmptyArrayContent(t *testing.T) {
 		t.Fatalf("empty tool_calls array must be dropped, got %s", om.ToolCalls)
 	}
 }
+
+// 上游报告输出里有多少是思考 token；缺少这个字段，客户端无法判断
+// completion_tokens 里多少是可见回答、多少是思考（两者都按输出计费）。
+func TestTokenUsageMarshalsCompletionTokensDetails(t *testing.T) {
+	u := TokenUsage{
+		Prompt: 37, Completion: 40, Total: 77,
+		CompletionTokensDetails: &CompletionTokensDetails{ReasoningTokens: 40},
+	}
+	b, err := json.Marshal(u)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var got map[string]interface{}
+	if err := json.Unmarshal(b, &got); err != nil {
+		t.Fatal(err)
+	}
+	ctd, ok := got["completion_tokens_details"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("completion_tokens_details 未输出: %s", b)
+	}
+	if n, _ := ctd["reasoning_tokens"].(float64); int(n) != 40 {
+		t.Errorf("reasoning_tokens = %v, want 40 (%s)", ctd["reasoning_tokens"], b)
+	}
+	// 标准 OpenAI 字段同时存在，老客户端不受影响。
+	if _, ok := got["completion_tokens"]; !ok {
+		t.Errorf("标准 completion_tokens 丢失: %s", b)
+	}
+
+	// 未设置时不得凭空造字段。
+	b2, _ := json.Marshal(TokenUsage{Prompt: 1, Completion: 2, Total: 3})
+	if strings.Contains(string(b2), "completion_tokens_details") {
+		t.Errorf("无数据时不应输出 completion_tokens_details: %s", b2)
+	}
+}