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.

common/gin.go

@@ -243,7 +243,15 @@ func ParseMultipartFormReusable(c *gin.Context) (*multipart.Form, error) {
 		return nil, err
 	}
 
-	contentType := c.Request.Header.Get("Content-Type")
+	// Use the original Content-Type saved on first call to avoid boundary
+	// mismatch when callers overwrite c.Request.Header after multipart rebuild.
+	var contentType string
+	if saved, ok := c.Get("_original_multipart_ct"); ok {
+		contentType = saved.(string)
+	} else {
+		contentType = c.Request.Header.Get("Content-Type")
+		c.Set("_original_multipart_ct", contentType)
+	}
 	boundary, err := parseBoundary(contentType)
 	if err != nil {
 		return nil, err