fix(acceptance): 隔离容量压测并稳定数据库连接

将协议模拟验收的供应商并发限额与 Worker 容量门禁分离，真实金丝雀继续保留生产限流。readyz 改用关键连接池并预热关键及 River 池，延长生产连接空闲周期，降低跨地域连接抖动。失败 Run 现在可以原子替换且失败报告记录任务数量，避免 validation 之间短暂放开正式流量。\n\n验证：Go 全量测试、go vet、PostgreSQL 集成测试、ShellCheck、迁移安全、发布脚本、pnpm lint/test/build、OpenAPI 无漂移均通过。

apps/api/internal/httpapi/availability_timeout_integration_test.go

@@ -35,6 +35,24 @@ func TestReadyReturnsPostgresUnavailableWithinTwoSeconds(t *testing.T) {
 	}
 }
 
+func TestReadyUsesReservedCriticalPoolWhenExecutionPoolIsBusy(t *testing.T) {
+	executionDB := newExhaustedPostgresStore(t)
+	criticalDB := newAvailablePostgresStore(t)
+	server := &Server{
+		store:             executionDB,
+		coordinationStore: criticalDB,
+		logger:            slog.New(slog.NewJSONHandler(io.Discard, nil)),
+	}
+	request := httptest.NewRequest(http.MethodGet, "/readyz", nil)
+	recorder := httptest.NewRecorder()
+
+	server.ready(recorder, request)
+
+	if recorder.Code != http.StatusOK {
+		t.Fatalf("readiness status=%d, want 200 while critical pool is available; body=%s", recorder.Code, recorder.Body.String())
+	}
+}
+
 func TestLoginReturnsAuthStoreUnavailableWithinFiveSeconds(t *testing.T) {
 	db := newExhaustedPostgresStore(t)
 	var logs bytes.Buffer
@@ -93,6 +111,31 @@ func newExhaustedPostgresStore(t *testing.T) *store.Store {
 	return db
 }
 
+func newAvailablePostgresStore(t *testing.T) *store.Store {
+	t.Helper()
+	databaseURL := strings.TrimSpace(os.Getenv("AI_GATEWAY_TEST_DATABASE_URL"))
+	if databaseURL == "" {
+		t.Skip("set AI_GATEWAY_TEST_DATABASE_URL to run PostgreSQL availability timeout tests")
+	}
+	parsed, err := url.Parse(databaseURL)
+	if err != nil {
+		t.Fatalf("parse test database URL: %v", err)
+	}
+	query := parsed.Query()
+	query.Set("pool_max_conns", "1")
+	query.Set("pool_min_conns", "0")
+	parsed.RawQuery = query.Encode()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	db, err := store.Connect(ctx, parsed.String())
+	if err != nil {
+		t.Fatalf("connect available test store: %v", err)
+	}
+	t.Cleanup(db.Close)
+	return db
+}
+
 func assertUnavailableResponse(t *testing.T, recorder *httptest.ResponseRecorder, expectedCode, expectedMessage string) {
 	t.Helper()
 	if recorder.Code != http.StatusServiceUnavailable {