fix(gateway): 剥离 Cursor raw body 透传路径中 Codex 不支持的 Responses API 参数

在前一个 commit 的 isResponsesShape 短路路径基础上,补充对 Cursor 云端
带过来的、Codex 上游统一不支持的顶层 Responses API 参数的剥离:

  - prompt_cache_retention
  - safety_identifier
  - metadata
  - stream_options

根因补充:这条 raw-body 透传路径为了保留 Cursor 的 input 数组整体结构,
不再经过 ChatCompletionsRequest 的反序列化过滤,所以这些 Go 结构体里
没有对应字段的参数会被原样发到上游,上游返回:
    Unsupported parameter: <field>
常规 Chat Completions 转换路径天然通过 ChatCompletionsRequest 丢弃未知字段,
不受影响;此处仅在 isResponsesShape 分支内用 sjson.DeleteBytes 显式过滤,
作用域最小。剥离列表与 openai_gateway_service.go:2034 的
unsupportedFields 语义对齐。

另外在 applyCodexOAuthTransform 的 OAuth 兜底 strip 列表里同步追加
prompt_cache_retention,作为对该函数所有其他 OAuth 调用点的 defense
in depth(当前只有 Cursor 路径的短路已在前面剥过,但保留这一层更稳)。

测试:
- TestCursorMixedShape_StripsUnsupportedFields — 验证所有 4 个字段都被剥
- TestApplyCodexOAuthTransform_StripsPromptCacheRetention — OAuth 兜底路径

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/openai_gateway_chat_completions.go

@@ -21,6 +21,22 @@ import (
 	"go.uber.org/zap"
 )
 
+// cursorResponsesUnsupportedFields are top-level Responses API parameters that
+// Codex upstreams reject with "Unsupported parameter: ...". They must be
+// stripped when forwarding a raw client body through the Responses-shape
+// short-circuit in ForwardAsChatCompletions (see isResponsesShape branch).
+// The normal Chat Completions → Responses conversion path is unaffected
+// because ChatCompletionsRequest has no fields for these parameters — unknown
+// fields are dropped naturally by json.Unmarshal. Kept semantically in sync
+// with the list in openai_gateway_service.go:2034 used by the /v1/responses
+// passthrough path.
+var cursorResponsesUnsupportedFields = []string{
+	"prompt_cache_retention",
+	"safety_identifier",
+	"metadata",
+	"stream_options",
+}
+
 // ForwardAsChatCompletions accepts a Chat Completions request body, converts it
 // to OpenAI Responses API format, forwards to the OpenAI upstream, and converts
 // the response back to Chat Completions format. All account types (OAuth and API
@@ -81,6 +97,16 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 		if err != nil {
 			return nil, fmt.Errorf("rewrite model in responses-shape body: %w", err)
 		}
+		// Strip Responses API parameters that no Codex upstream accepts.
+		// Because this branch forwards the raw body (the normal path rebuilds
+		// it from ChatCompletionsRequest and drops unknown fields naturally),
+		// we must filter these fields explicitly here — otherwise the upstream
+		// rejects the request with "Unsupported parameter: ...".
+		for _, field := range cursorResponsesUnsupportedFields {
+			if stripped, derr := sjson.DeleteBytes(responsesBody, field); derr == nil {
+				responsesBody = stripped
+			}
+		}
 		// Minimal stub populated from the raw body so downstream billing
 		// propagation (ServiceTier, ReasoningEffort) keeps working.
 		responsesReq = &apicompat.ResponsesRequest{