fix: keep begin_checkout user_id reactive in subscription flows (#8726)

## Summary

Use reactive `userId` reads for `begin_checkout` telemetry so delayed
auth state updates are reflected at event time instead of using a stale
snapshot.

## Changes

- **What**: switched subscription checkout telemetry paths to
`storeToRefs(useFirebaseAuthStore())` and read `userId.value` when
dispatching `trackBeginCheckout`.
- **What**: added regression tests that mutate `userId` after setup /
after checkout starts and assert telemetry uses the updated ID.

## Review Focus

- Verify `PricingTable` and `performSubscriptionCheckout` still emit
exactly one `begin_checkout` event per action, with `checkout_type:
change` and `checkout_type: new` in their respective paths.
- Verify the new tests would fail with stale store destructuring
(manually validated during development).

## Screenshots (if applicable)

N/A

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8726-fix-keep-begin_checkout-user_id-reactive-in-subscription-flows-3006d73d365081888c84c0335ab52e09)
by [Unito](https://www.unito.io)

src/platform/cloud/subscription/components/PricingTable.vue

@@ -243,6 +243,7 @@
 
 <script setup lang="ts">
 import { cn } from '@comfyorg/tailwind-utils'
+import { storeToRefs } from 'pinia'
 import Popover from 'primevue/popover'
 import SelectButton from 'primevue/selectbutton'
 import type { ToggleButtonPassThroughMethodOptions } from 'primevue/togglebutton'
@@ -333,7 +334,7 @@ const tiers: PricingTierConfig[] = [
 const { isActiveSubscription, subscriptionTier, isYearlySubscription } =
   useSubscription()
 const telemetry = useTelemetry()
-const { userId } = useFirebaseAuthStore()
+const { userId } = storeToRefs(useFirebaseAuthStore())
 const { accessBillingPortal, reportError } = useFirebaseAuthActions()
 const { wrapWithErrorHandlingAsync } = useErrorHandling()
 
@@ -415,9 +416,9 @@ const handleSubscribe = wrapWithErrorHandlingAsync(
     try {
       if (isActiveSubscription.value) {
         const checkoutAttribution = getCheckoutAttribution()
-        if (userId) {
+        if (userId.value) {
           telemetry?.trackBeginCheckout({
-            user_id: userId,
+            user_id: userId.value,
             tier: tierKey,
             cycle: currentBillingCycle.value,
             checkout_type: 'change',