refactor: 重构 Chat Completions 端点，采用类型安全的 Responses API 转换

将 /v1/chat/completions 端点从 ResponseWriter 劫持模式重构为独立的
类型安全转换路径，与 Anthropic Messages 端点架构对齐：

- 在 apicompat 包新增 Chat Completions 完整类型定义和双向转换器
- 新增 ForwardAsChatCompletions service 方法，走 Responses API 上游
- Handler 改为独立的账号选择/failover 循环，不再劫持 Responses handler
- 提取 handleCompatErrorResponse 为 Chat Completions 和 Messages 共用
- 删除旧的 forwardChatCompletions 直传路径及相关死代码

backend/internal/service/openai_gateway_service.go

@@ -12,7 +12,6 @@ import (
 	"io"
 	"math/rand"
 	"net/http"
-	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -37,7 +36,6 @@ const (
 	chatgptCodexURL = "https://chatgpt.com/backend-api/codex/responses"
 	// OpenAI Platform API for API Key accounts (fallback)
 	openaiPlatformAPIURL   = "https://api.openai.com/v1/responses"
-	openaiChatAPIURL       = "https://api.openai.com/v1/chat/completions"
 	openaiStickySessionTTL = time.Hour // 粘性会话TTL
 	codexCLIUserAgent      = "codex_cli_rs/0.104.0"
 	// codex_cli_only 拒绝时单个请求头日志长度上限（字符）
@@ -56,16 +54,6 @@ const (
 	codexCLIVersion                    = "0.104.0"
 )
 
-// OpenAIChatCompletionsBodyKey stores the original chat-completions payload in gin.Context.
-const OpenAIChatCompletionsBodyKey = "openai_chat_completions_body"
-
-// OpenAIChatCompletionsIncludeUsageKey stores stream_options.include_usage in gin.Context.
-const OpenAIChatCompletionsIncludeUsageKey = "openai_chat_completions_include_usage"
-
-// openaiSSEDataRe matches SSE data lines with optional whitespace after colon.
-// Some upstream APIs return non-standard "data:" without space (should be "data: ").
-var openaiSSEDataRe = regexp.MustCompile(`^data:\s*`)
-
 // OpenAI allowed headers whitelist (for non-passthrough).
 var openaiAllowedHeaders = map[string]bool{
 	"accept-language":       true,
@@ -109,19 +97,6 @@ var codexCLIOnlyDebugHeaderWhitelist = []string{
 	"X-Real-IP",
 }
 
-// OpenAI chat-completions allowed headers (extend responses whitelist).
-var openaiChatAllowedHeaders = map[string]bool{
-	"accept-language":     true,
-	"content-type":        true,
-	"conversation_id":     true,
-	"user-agent":          true,
-	"originator":          true,
-	"session_id":          true,
-	"openai-organization": true,
-	"openai-project":      true,
-	"openai-beta":         true,
-}
-
 // OpenAICodexUsageSnapshot represents Codex API usage limits from response headers
 type OpenAICodexUsageSnapshot struct {
 	PrimaryUsedPercent          *float64 `json:"primary_used_percent,omitempty"`
@@ -1602,23 +1577,6 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		return nil, errors.New("codex_cli_only restriction: only codex official clients are allowed")
 	}
 
-	if c != nil && account != nil && account.Type == AccountTypeAPIKey {
-		if raw, ok := c.Get(OpenAIChatCompletionsBodyKey); ok {
-			if rawBody, ok := raw.([]byte); ok && len(rawBody) > 0 {
-				includeUsage := false
-				if v, ok := c.Get(OpenAIChatCompletionsIncludeUsageKey); ok {
-					if flag, ok := v.(bool); ok {
-						includeUsage = flag
-					}
-				}
-				if passthroughWriter, ok := c.Writer.(interface{ SetPassthrough() }); ok {
-					passthroughWriter.SetPassthrough()
-				}
-				return s.forwardChatCompletions(ctx, c, account, rawBody, includeUsage, startTime)
-			}
-		}
-	}
-
 	originalBody := body
 	reqModel, reqStream, promptCacheKey := extractOpenAIRequestMetaFromBody(body)
 	originalModel := reqModel
@@ -2989,6 +2947,120 @@ func (s *OpenAIGatewayService) handleErrorResponse(
 	return nil, fmt.Errorf("upstream error: %d message=%s", resp.StatusCode, upstreamMsg)
 }
 
+// compatErrorWriter is the signature for format-specific error writers used by
+// the compat paths (Chat Completions and Anthropic Messages).
+type compatErrorWriter func(c *gin.Context, statusCode int, errType, message string)
+
+// handleCompatErrorResponse is the shared non-failover error handler for the
+// Chat Completions and Anthropic Messages compat paths. It mirrors the logic of
+// handleErrorResponse (passthrough rules, ShouldHandleErrorCode, rate-limit
+// tracking, secondary failover) but delegates the final error write to the
+// format-specific writer function.
+func (s *OpenAIGatewayService) handleCompatErrorResponse(
+	resp *http.Response,
+	c *gin.Context,
+	account *Account,
+	writeError compatErrorWriter,
+) (*OpenAIForwardResult, error) {
+	body, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
+
+	upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(body))
+	if upstreamMsg == "" {
+		upstreamMsg = fmt.Sprintf("Upstream error: %d", resp.StatusCode)
+	}
+	upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+
+	upstreamDetail := ""
+	if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
+		maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
+		if maxBytes <= 0 {
+			maxBytes = 2048
+		}
+		upstreamDetail = truncateString(string(body), maxBytes)
+	}
+	setOpsUpstreamError(c, resp.StatusCode, upstreamMsg, upstreamDetail)
+
+	// Apply error passthrough rules
+	if status, errType, errMsg, matched := applyErrorPassthroughRule(
+		c, account.Platform, resp.StatusCode, body,
+		http.StatusBadGateway, "api_error", "Upstream request failed",
+	); matched {
+		writeError(c, status, errType, errMsg)
+		if upstreamMsg == "" {
+			upstreamMsg = errMsg
+		}
+		if upstreamMsg == "" {
+			return nil, fmt.Errorf("upstream error: %d (passthrough rule matched)", resp.StatusCode)
+		}
+		return nil, fmt.Errorf("upstream error: %d (passthrough rule matched) message=%s", resp.StatusCode, upstreamMsg)
+	}
+
+	// Check custom error codes — if the account does not handle this status,
+	// return a generic error without exposing upstream details.
+	if !account.ShouldHandleErrorCode(resp.StatusCode) {
+		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+			Platform:           account.Platform,
+			AccountID:          account.ID,
+			AccountName:        account.Name,
+			UpstreamStatusCode: resp.StatusCode,
+			UpstreamRequestID:  resp.Header.Get("x-request-id"),
+			Kind:               "http_error",
+			Message:            upstreamMsg,
+			Detail:             upstreamDetail,
+		})
+		writeError(c, http.StatusInternalServerError, "api_error", "Upstream gateway error")
+		if upstreamMsg == "" {
+			return nil, fmt.Errorf("upstream error: %d (not in custom error codes)", resp.StatusCode)
+		}
+		return nil, fmt.Errorf("upstream error: %d (not in custom error codes) message=%s", resp.StatusCode, upstreamMsg)
+	}
+
+	// Track rate limits and decide whether to trigger secondary failover.
+	shouldDisable := false
+	if s.rateLimitService != nil {
+		shouldDisable = s.rateLimitService.HandleUpstreamError(
+			c.Request.Context(), account, resp.StatusCode, resp.Header, body,
+		)
+	}
+	kind := "http_error"
+	if shouldDisable {
+		kind = "failover"
+	}
+	appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+		Platform:           account.Platform,
+		AccountID:          account.ID,
+		AccountName:        account.Name,
+		UpstreamStatusCode: resp.StatusCode,
+		UpstreamRequestID:  resp.Header.Get("x-request-id"),
+		Kind:               kind,
+		Message:            upstreamMsg,
+		Detail:             upstreamDetail,
+	})
+	if shouldDisable {
+		return nil, &UpstreamFailoverError{
+			StatusCode:             resp.StatusCode,
+			ResponseBody:           body,
+			RetryableOnSameAccount: account.IsPoolMode() && isPoolModeRetryableStatus(resp.StatusCode),
+		}
+	}
+
+	// Map status code to error type and write response
+	errType := "api_error"
+	switch {
+	case resp.StatusCode == 400:
+		errType = "invalid_request_error"
+	case resp.StatusCode == 404:
+		errType = "not_found_error"
+	case resp.StatusCode == 429:
+		errType = "rate_limit_error"
+	case resp.StatusCode >= 500:
+		errType = "api_error"
+	}
+
+	writeError(c, resp.StatusCode, errType, upstreamMsg)
+	return nil, fmt.Errorf("upstream error: %d %s", resp.StatusCode, upstreamMsg)
+}
+
 // openaiStreamingResult streaming response result
 type openaiStreamingResult struct {
 	usage        *OpenAIUsage