feat(gemini): 接入 Veo 官方异步生成协议

将 Gemini Veo 视频请求改为 predictLongRunning 提交与 Operation 轮询，支持任务恢复、首尾帧和参考图的官方 Base64 字段映射。

完成后由网关携带 API Key 从受信任的 Gemini 文件域下载视频，再进入现有对象存储转存链路，避免暴露鉴权地址。

验证：env -u AI_GATEWAY_TEST_DATABASE_URL go test ./... -count=1；go vet ./...；真实 Veo 3.1 任务生成并下载 1633544 字节视频。

apps/api/internal/runner/request_assets.go

@@ -164,7 +164,7 @@ func (s *Service) hydrateProviderRequestAssetString(ctx context.Context, value s
 		return value, nil
 	}
 	style, ok := requestAssetHydrateUnsupported, false
-	if openAIImageEditRequiresMultipartBytes(candidate) {
+	if geminiVeoRequiresInlineImage(candidate) || openAIImageEditRequiresMultipartBytes(candidate) {
 		style, ok = requestAssetHydrateDataURL, true
 	} else {
 		style, ok = requestAssetCapabilityHydrationForMedia("image", candidate, raw, "")
@@ -395,6 +395,9 @@ func requestAssetHydrationForField(path []string, asset store.RequestAsset, cand
 		return requestAssetHydrateDataURL
 	}
 	if requestAssetMediaKindForHydration(path, asset) == "image" {
+		if geminiVeoRequiresInlineImage(candidate) {
+			return requestAssetHydrateDataURL
+		}
 		if openAIImageEditRequiresMultipartBytes(candidate) {
 			return requestAssetHydrateDataURL
 		}
@@ -418,6 +421,11 @@ func openAIImageEditRequiresMultipartBytes(candidate store.RuntimeModelCandidate
 		strings.TrimSpace(candidate.ModelType) == "image_edit"
 }
 
+func geminiVeoRequiresInlineImage(candidate store.RuntimeModelCandidate) bool {
+	return normalizeProviderKey(candidate.Provider) == "gemini" &&
+		strings.Contains(strings.ToLower(strings.TrimSpace(firstNonEmptyString(candidate.ProviderModelName, candidate.ModelName))), "veo")
+}
+
 func requestAssetMediaKindForHydration(path []string, asset store.RequestAsset) string {
 	if mediaURLFieldNeedsHydration(path) {
 		return requestAssetMediaURLKind(path)