feat(task): add model redirection, per-call billing, and multipart retry fix for async tasks

1. Async task model redirection (aligned with sync tasks):
   - Integrate ModelMappedHelper in RelayTaskSubmit after model name
     determination, populating OriginModelName / UpstreamModelName on RelayInfo.
   - All task adaptors now send UpstreamModelName to upstream providers:
     - Gemini & Vertex: BuildRequestURL uses UpstreamModelName.
     - Doubao & Ali: BuildRequestBody conditionally overwrites body.Model.
     - Vidu, Kling, Hailuo, Jimeng: convertToRequestPayload accepts RelayInfo
       and unconditionally uses info.UpstreamModelName.
     - Sora: BuildRequestBody parses JSON and multipart bodies to replace
       the "model" field with UpstreamModelName.
   - Frontend log visibility: LogTaskConsumption and taskBillingOther now
     emit is_model_mapped / upstream_model_name in the "other" JSON field.
   - Billing safety: RecalculateTaskQuotaByTokens reads model name from
     BillingContext.OriginModelName (via taskModelName) instead of
     task.Data["model"], preventing billing leaks from upstream model names.

2. Per-call billing (TaskPricePatches lifecycle):
   - Rename TaskBillingContext.ModelName → OriginModelName; add PerCallBilling
     bool field, populated from TaskPricePatches at submission time.
   - settleTaskBillingOnComplete short-circuits when PerCallBilling is true,
     skipping both adaptor adjustments and token-based recalculation.
   - Remove ModelName from TaskSubmitResult; use relayInfo.OriginModelName
     consistently in controller/relay.go for billing context and logging.

3. Multipart retry boundary mismatch fix:
   - Root cause: after Sora (or OpenAI audio) rebuilds a multipart body with a
     new boundary and overwrites c.Request.Header["Content-Type"], subsequent
     calls to ParseMultipartFormReusable on retry would parse the cached
     original body with the wrong boundary, causing "NextPart: EOF".
   - Fix: ParseMultipartFormReusable now caches the original Content-Type in
     gin context key "_original_multipart_ct" on first call and reuses it for
     all subsequent parses, making multipart parsing retry-safe globally.
   - Sora adaptor reverted to the standard pattern (direct header set/get),
     which is now safe thanks to the root fix.

4. Tests:
   - task_billing_test.go: update makeTask to use OriginModelName; add
     PerCallBilling settlement tests (skip adaptor adjust, skip token recalc);
     add non-per-call adaptor adjustment test with refund verification.

service/task_polling.go

@@ -467,6 +467,11 @@ func truncateBase64(s string) string {
 //  2. taskResult.TotalTokens > 0 → 按 token 重算
 //  3. 都不满足 → 保持预扣额度不变
 func settleTaskBillingOnComplete(ctx context.Context, adaptor TaskPollingAdaptor, task *model.Task, taskResult *relaycommon.TaskInfo) {
+	// 0. 按次计费的任务不做差额结算
+	if bc := task.PrivateData.BillingContext; bc != nil && bc.PerCallBilling {
+		logger.LogInfo(ctx, fmt.Sprintf("任务 %s 按次计费，跳过差额结算", task.TaskID))
+		return
+	}
 	// 1. 优先让 adaptor 决定最终额度
 	if actualQuota := adaptor.AdjustBillingOnComplete(task, taskResult); actualQuota > 0 {
 		RecalculateTaskQuota(ctx, task, actualQuota, "adaptor计费调整")