Cache web purchase redemption CustomerInfo for the initiating App User ID (#4114)

## Description

After making the API request to POST redeem web purchase, and receiving
the response with the updated `CustomerInfo`, we reached back into the
identityManager to get the current appUserID in order to cache the
updated `CustomerInfo` for this app user ID. However theoretically the
user ID could have changed between initiating the request and receiving
the response. Possibly saving the updated `CustomerInfo` of user A under
the cache key for user `B`, resulting in `CustomerInfo` data being
shared between app user IDs

## Changes

We're now capturing the initiating `appUserID` and reusing that value
when storing the `CustomerInfo` in the cache. Ensuring that it's saved
for the app user that originally initiated the request.

Equivalent iOS PR: https://github.com/RevenueCat/purchases-ios/pull/7533

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---

> [!NOTE]
> **Medium Risk**
> Changes customer-info cache keying for web redemption, which could
affect entitlement data if wrong; the fix is narrowly scoped and covered
by a regression test.
> 
> **Overview**
> Fixes a race where **web purchase redemption** could cache updated
`CustomerInfo` under the wrong app user if
`identityManager.currentAppUserID` changed between starting the redeem
request and handling the async success.
> 
> `CustomerInfoUpdateHandler` now exposes
`cacheAndNotifyListeners(customerInfo, appUserID)`; the existing
single-argument method delegates to the current ID.
`WebPurchaseRedemptionHelper` captures `appUserID` when redemption
starts and passes that same ID when caching after success.
> 
> Tests were updated to expect the explicit user ID, plus a new case
that simulates a user switch during the async callback and verifies
cache writes use the **original** user ID.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
817d1221db7bde5304029a640d932c8862911650. Bugbot is set up for automated
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<!-- /CURSOR_SUMMARY -->