fix(billing): don't crash when workspace has no active subscription (#21510)

## Problem

Sentry (high severity, SLA-breaching): `Billing Subscription Not Found:
No active subscription found for workspace …`

The `billingSubscription` workspace-cache provider
(`WorkspaceBillingSubscriptionCacheService.computeForCache`) called
`getCurrentBillingSubscriptionOrThrow`. For a workspace whose
subscription is fully canceled, `getCurrentBillingSubscription` filters
out `Canceled` and returns `undefined`, so the provider **threw**
`BILLING_SUBSCRIPTION_NOT_FOUND`.

That cache key is read on every usage-recording path:
- workflow execution
(`WorkflowExecutorWorkspaceService.sendWorkflowNodeRunEvent`)
- AI usage (`AiBillingService`)
- logic-function execution (`LogicFunctionExecutorService`)
- app charges (`AppBillingService`)
- the gate `BillingUsageService.canFeatureBeUsed` /
`hasAvailableCredits` / `decrementAvailableCreditsInCache`
- the cancellation webhook
(`invalidateAndRecompute('billingSubscription')`)

So any of these throws an unhandled exception for a
no-active-subscription workspace. The intent was clearly to tolerate
this state — `canFeatureBeUsed` already guards with
`isDefined(billingSubscription)` and the workflow runner logs *"there is
no subscription for this workspace"* — but the throwing provider made
those guards unreachable.

## Fix

- `computeForCache` now returns `FlatBillingSubscription | null` via the
non-throwing `getCurrentBillingSubscription`, and the cache type allows
`null`.
- Every consumer guards the absent case (`isDefined` / optional
chaining) and no-ops: usage events still emit with an undefined
`periodStart`, credits aren't decremented, `hasAvailableCredits` returns
`false`.
- `getCurrentBillingSubscriptionOrThrow` is **left untouched** for the
many callers (resolver, subscription-update, etc.) that genuinely
require a subscription.

## Test

Adds `workspace-billing-subscription-cache.service.spec.ts`: the
provider returns `null` when there's no active subscription (regression)
and the flattened subscription when one exists.

All 142 tests across the billing / ai-billing / workflow-executor suites
pass; `oxlint --type-aware` and `oxfmt` are clean.

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packages/twenty-server/src/modules/workflow/workflow-executor/workspace-services/__tests__/workflow-executor.workspace-service.spec.ts

@@ -126,7 +126,7 @@ describe('WorkflowExecutorWorkspaceService', () => {
           provide: WorkspaceCacheService,
           useValue: {
             getOrRecompute: jest.fn().mockResolvedValue({
-              billingSubscription: {
+              currentBillingSubscription: {
                 currentPeriodStart: new Date('2026-04-01T00:00:00Z'),
               },
             }),

packages/twenty-server/src/modules/workflow/workflow-executor/workspace-services/workflow-executor.workspace-service.ts

@@ -8,6 +8,7 @@ import {
   WorkflowRunStepInfos,
 } from 'twenty-shared/workflow';
 
+import { NO_BILLING_SUBSCRIPTION } from 'src/engine/core-modules/billing/constants/no-billing-subscription.constant';
 import { BillingUsageService } from 'src/engine/core-modules/billing/services/billing-usage.service';
 import { BillingService } from 'src/engine/core-modules/billing/services/billing.service';
 import { ExceptionHandlerService } from 'src/engine/core-modules/exception-handler/exception-handler.service';
@@ -327,18 +328,19 @@ export class WorkflowExecutorWorkspaceService {
   ) {
     let periodStart: Date | undefined;
     if (this.billingService.isBillingEnabled()) {
-      const {
-        billingSubscription: { currentPeriodStart },
-      } = await this.workspaceCacheService.getOrRecompute(workspaceId, [
-        'billingSubscription',
-      ]);
+      const { currentBillingSubscription } =
+        await this.workspaceCacheService.getOrRecompute(workspaceId, [
+          'currentBillingSubscription',
+        ]);
 
-      periodStart = currentPeriodStart;
+      if (currentBillingSubscription !== NO_BILLING_SUBSCRIPTION) {
+        periodStart = currentBillingSubscription.currentPeriodStart;
 
-      await this.billingUsageService.decrementAvailableCreditsInCache({
-        workspaceId,
-        usedCredits: 100,
-      });
+        await this.billingUsageService.decrementAvailableCreditsInCache({
+          workspaceId,
+          usedCredits: 100,
+        });
+      }
     }
 
     this.workspaceEventEmitter.emitCustomBatchEvent<UsageEvent>(