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_test.go

@@ -172,6 +172,24 @@ func TestHydrateProviderRequestAssetsConvertsGeminiInlineDataAssetToRawBase64(t
 	}
 }
 
+func TestGeminiVeoForcesOfficialInlineImageFormat(t *testing.T) {
+	candidate := store.RuntimeModelCandidate{
+		Provider:          "gemini",
+		ProviderModelName: "veo-3.1-generate-preview",
+		ModelType:         "image_to_video",
+		Capabilities: map[string]any{
+			"image_to_video": map[string]any{
+				"support_url_input":    true,
+				"support_base64_input": true,
+			},
+		},
+	}
+	asset := store.RequestAsset{URL: "https://cdn.example.com/input.png", StorageProvider: "remote", ContentType: "image/png"}
+	if got := requestAssetHydrationForField([]string{"image"}, asset, candidate); got != requestAssetHydrateDataURL {
+		t.Fatalf("Gemini Veo must hydrate public image URLs as data URLs, got %q", got)
+	}
+}
+
 func TestHydrateProviderRequestAssetsConvertsVolcesImageURLAssetToDataURL(t *testing.T) {
 	storageDir := t.TempDir()
 	fileName := "gateway-request-asset-chat-image.png"