AppPurchase: log underlying error when status is canceled

The in_app_purchase plugin maps several silent Google Play rejections
to PurchaseStatus.canceled — offer ineligibility, missing payment
method, region mismatch, billing-service hiccups all surface the same
way as a real user-initiated cancel. Without the underlying error
fields we can't tell those apart, which has been blocking root-cause
on a cluster of "I bought Pro but it didn't activate" tickets from
v9.1.x Android users in China (e.g. engineering#3463 follow-up
investigation of FD #174173, where a renewing Pro user sees 7+
consecutive `canceled` responses across 30 minutes).

Capture productID, error.code, error.message, and error.details on
the canceled branch. On Android `error.details` typically contains
a Map with the raw BillingClient response_code and debug_message,
which is the signal we need to differentiate real cancels from silent
Play rejections.

No behavior change — the user-visible "Purchase canceled" snackbar
fires exactly as before. Single info-level log line added.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Adam Fisk

lib/core/services/app_purchase.dart

@@ -323,7 +323,21 @@ class AppPurchase {
         return;
       }
       if (status == PurchaseStatus.canceled) {
-        /// User has cancelled the purchase
+        // `canceled` is not necessarily a real user cancel — the
+        // in_app_purchase plugin also maps several silent Google Play
+        // rejections (offer ineligibility, missing payment method, region
+        // mismatch, billing-service hiccups) to the same status. Without
+        // capturing the underlying error we can't distinguish those from a
+        // user who actually X'd the dialog, which is the difference between
+        // "expected" and "a bug to fix." For Android, `error.details`
+        // typically contains a Map with the raw BillingClient
+        // `response_code` and `debug_message`.
+        appLogger.info(
+          '[AppPurchase] Purchase canceled: productID=${purchaseDetails.productID}'
+          ' errorCode=${purchaseDetails.error?.code}'
+          ' message=${purchaseDetails.error?.message}'
+          ' details=${purchaseDetails.error?.details}',
+        );
         _onError?.call("Purchase canceled");
         return;
       }