fix(opencode): normalize the quota endpoint for the /zen/go base variant

OpenCode Go 有两个都合法的 base_url：https://opencode.ai/zen/go/v1（OpenAI
协议档，DefaultOpenCodeGoBaseURL）与 https://opencode.ai/zen/go（Anthropic
协议档，前端 defaultCNBaseUrl 会写入）。旧拼接只做 strings.TrimRight(base,
"/") + "/usage"，Anthropic 档账号的额度探测必然打到 /zen/go/usage。

实测证据（curl 无鉴权，401=端点存在，404=不存在）：
  401 https://opencode.ai/zen/go/v1/usage
  404 https://opencode.ai/zen/go/usage

修法沿用同包 kimiQuotaURL（coding/v1 与 coding 两种 base）的既有惯例：
TrimSuffix(base, "/v1") 后统一拼回 /v1/usage，两个变体与尾斜杠写法都归一到
https://opencode.ai/zen/go/v1/usage；自定义 base 保持同一语义。

backend/internal/service/opencode_go.go

@@ -322,10 +322,16 @@ func (a *Account) ResolveOpenCodeGoUpstreamProtocol(model string) string {
 	}
 }
 
+// openCodeGoQuotaURL 根据 base_url 解析 OpenCode Go 额度端点。
+// /zen/go/v1（Chat 协议默认，DefaultOpenCodeGoBaseURL）与 /zen/go
+// （Anthropic 协议默认，DefaultOpenCodeGoAnthropicBaseURL）两种 base 统一
+// 剥掉尾部 /v1 后拼回 /v1/usage（实测 /zen/go/usage → 404），协议切换不
+// 影响额度探测端点。与 kimiQuotaURL 同一惯例。
 func openCodeGoQuotaURL(baseURL string) string {
 	base := strings.TrimRight(strings.TrimSpace(baseURL), "/")
 	if base == "" {
 		base = DefaultOpenCodeGoBaseURL
 	}
-	return base + openCodeGoUsagePath
+	base = strings.TrimSuffix(base, "/v1")
+	return base + "/v1" + openCodeGoUsagePath
 }

backend/internal/service/opencode_go_test.go

@@ -205,9 +205,26 @@ func TestNormalizeOpenCodeGoProtocolRulesCredentials(t *testing.T) {
 
 func TestOpenCodeGoQuotaURL(t *testing.T) {
 	t.Parallel()
-	require.Equal(t, "https://opencode.ai/zen/go/v1/usage", openCodeGoQuotaURL(""))
-	require.Equal(t, "https://opencode.ai/zen/go/v1/usage", openCodeGoQuotaURL(DefaultOpenCodeGoBaseURL+"/"))
-	require.Equal(t, "https://custom.example/v1/usage", openCodeGoQuotaURL("https://custom.example/v1"))
+	const want = "https://opencode.ai/zen/go/v1/usage"
+	cases := []struct {
+		name    string
+		baseURL string
+		want    string
+	}{
+		{"empty falls back to default", "", want},
+		{"chat base (/v1)", DefaultOpenCodeGoBaseURL, want},
+		{"chat base trailing slash", DefaultOpenCodeGoBaseURL + "/", want},
+		{"anthropic base (no /v1)", DefaultOpenCodeGoAnthropicBaseURL, want},
+		{"anthropic base trailing slash", DefaultOpenCodeGoAnthropicBaseURL + "/", want},
+		{"zen base stays idempotent", DefaultOpenCodeZenBaseURL, "https://opencode.ai/zen/v1/usage"},
+		{"custom base with /v1", "https://custom.example/v1", "https://custom.example/v1/usage"},
+		{"custom base with /v1 trailing slash", "https://custom.example/v1/", "https://custom.example/v1/usage"},
+		// 与 kimiQuotaURL 同语义：无条件补 /v1，非官方域名同样归一。
+		{"custom base without /v1", "https://custom.example", "https://custom.example/v1/usage"},
+	}
+	for _, tc := range cases {
+		require.Equal(t, tc.want, openCodeGoQuotaURL(tc.baseURL), "case=%s", tc.name)
+	}
 }
 
 func TestDefaultOpenCodeGoModelIDsCoverDocumentedCatalog(t *testing.T) {