Add Integration and unit tests on Billing (#9317)

Solves [ https://github.com/twentyhq/private-issues/issues/214 ]

**TLDR**
Add unit and integration tests to Billing. First approach to run jest
integration tests directly from VSCode.

**In order to run the unit tests:**
Run unit test using the CLI or with the jest extension directly from
VSCode.

**In order to run the integration tests:**
Ensure that your database has the billingTables. If that's not the case,
migrate the database with IS_BILLING_ENABLED set to true:
` npx nx run twenty-server:test:integration
test/integration/billing/suites/billing-controller.integration-spec.ts`

**Doing:**
- Unit test on transformSubscriptionEventToSubscriptionItem
- More tests cases in billingController integration tests.

---------

Co-authored-by: Félix Malfait <felix.malfait@gmail.com>
Co-authored-by: Weiko <corentin@twenty.com>
Co-authored-by: Charles Bochet <charlesBochet@users.noreply.github.com>

packages/twenty-server/test/integration/billing/utils/create-mock-stripe-price-created-data.util.ts

@@ -0,0 +1,34 @@
+import Stripe from 'stripe';
+export const createMockStripePriceCreatedData = (
+  overrides = {},
+): Stripe.PriceCreatedEvent.Data => ({
+  object: {
+    id: 'price_1Q',
+    object: 'price',
+    active: true,
+    billing_scheme: 'per_unit',
+    created: 1733734326,
+    currency: 'usd',
+    custom_unit_amount: null,
+    livemode: false,
+    lookup_key: null,
+    metadata: {},
+    nickname: null,
+    product: 'prod_RLN',
+    recurring: {
+      aggregate_usage: null,
+      interval: 'month',
+      interval_count: 1,
+      meter: null,
+      trial_period_days: null,
+      usage_type: 'licensed',
+    },
+    tax_behavior: 'unspecified',
+    tiers_mode: null,
+    transform_quantity: null,
+    type: 'recurring',
+    unit_amount: 0,
+    unit_amount_decimal: '0',
+    ...overrides,
+  },
+});