fix(billing): serialize error as message+stack in 502 log payloads

Winston format.json() does not serialize Error non-enumerable properties,
so logging `error: err` produces `{}` in JSON mode. Switch to
`error: err?.message ?? String(err), stack: err?.stack` to match the
existing convention in billing.webhook.service.js. Update regression test
assertions to verify the extracted message string.

Author: PierreBrisorgueil

modules/billing/controllers/billing.controller.js

@@ -27,7 +27,8 @@ const checkout = async (req, res) => {
     const status = err.message?.startsWith('Invalid') || err.message?.includes('not found') ? 422 : 502;
     if (status === 502) {
       logger.error('[billing.checkout] createCheckout failed', {
-        error: err,
+        error: err?.message ?? String(err),
+        stack: err?.stack,
         organizationId: req.organization?._id,
         priceId: req.body?.priceId,
         source: 'web',
@@ -149,7 +150,8 @@ const extrasCheckout = async (req, res) => {
     const status = err.message?.startsWith('Invalid') || err.message?.includes('not found') ? 422 : 502;
     if (status === 502) {
       logger.error('[billing.checkout] createExtrasCheckout failed', {
-        error: err,
+        error: err?.message ?? String(err),
+        stack: err?.stack,
         organizationId: req.organization?._id,
         packId: req.body?.packId,
         source: 'web',

modules/billing/services/billing.service.js

@@ -58,7 +58,8 @@ const _ensureStripeCustomer = async (stripe, organization) => {
     );
   } catch (err) {
     logger.error('[billing.service] stripe.customers.create failed', {
-      error: err,
+      error: err?.message ?? String(err),
+      stack: err?.stack,
       organizationId: String(organization._id),
     });
     throw err;
@@ -205,7 +206,8 @@ const createCheckout = async (organization, priceId, successUrl, cancelUrl) => {
     session = await stripe.checkout.sessions.create(checkoutParams);
   } catch (err) {
     logger.error('[billing.service] stripe.checkout.sessions.create failed', {
-      error: err,
+      error: err?.message ?? String(err),
+      stack: err?.stack,
       organizationId: String(organization._id),
       priceId,
       plan: matchedPlan.planId,
@@ -299,7 +301,8 @@ const createExtrasCheckout = async (organization, packId, successUrl, cancelUrl,
     );
   } catch (err) {
     logger.error('[billing.service] stripe.checkout.sessions.create (extras) failed', {
-      error: err,
+      error: err?.message ?? String(err),
+      stack: err?.stack,
       organizationId: orgId,
       packId,
     });

modules/billing/tests/billing.checkout.unit.tests.js

@@ -873,7 +873,10 @@ describe('Billing service unit tests:', () => {
 
       expect(mockLogger.error).toHaveBeenCalledWith(
         '[billing.service] stripe.customers.create failed',
-        expect.objectContaining({ organizationId: orgId }),
+        expect.objectContaining({
+          organizationId: orgId,
+          error: 'Stripe Test: customer creation blocked',
+        }),
       );
     });
 
@@ -897,7 +900,11 @@ describe('Billing service unit tests:', () => {
 
       expect(mockLogger.error).toHaveBeenCalledWith(
         '[billing.service] stripe.checkout.sessions.create failed',
-        expect.objectContaining({ organizationId: orgId, priceId: 'price_starter_m' }),
+        expect.objectContaining({
+          organizationId: orgId,
+          priceId: 'price_starter_m',
+          error: 'Stripe Test: session creation blocked',
+        }),
       );
     });
 
@@ -929,7 +936,11 @@ describe('Billing service unit tests:', () => {
 
       expect(mockLogger.error).toHaveBeenCalledWith(
         '[billing.service] stripe.checkout.sessions.create (extras) failed',
-        expect.objectContaining({ organizationId: orgId, packId: 'pack_500k' }),
+        expect.objectContaining({
+          organizationId: orgId,
+          packId: 'pack_500k',
+          error: 'Stripe Test: extras session blocked',
+        }),
       );
     });
   });