Purchase completion opens the AI gate from a session captured before the StoreKit await

[tabmail-kmyi]

Summary

Purchase completion opens the process-global AI gate based on a session that was captured before the StoreKit await. If the signed-in account changes during that window, the gate can be opened while a different account is current. This is the purchase-path sibling of the restore-path ownership guard added in #50 (which was intentionally scoped to the restore path only).

• Pre-existing on main — not introduced by the signup-trial work (Support server-granted free trial in account and paywall UI #49 / Do not open the AI gate when restoring a different account's subscription #50).
• Severity: low — degenerate (requires a same-device account switch mid-purchase) and self-healing.

Mechanism

PlanPickerView.purchaseProduct(_:):

1. captures session = TabMailAuthService.getSession() before await storeKit.purchase(...),
2. after the await resolves, calls AISubscriptionGate.shared.openGate().

AISubscriptionGate is process-global, not per-account. The transaction is correctly stamped with the purchaser's appAccountToken, but the local gate is opened for whichever account is current when the await resolves. If the app switched from account A → B during the purchase, B's gate opens. It self-heals on B's next authoritative whoami (AISubscriptionGate.apply() closes on has_subscription:false).

Proposed fix (common-path-safe)

At purchase completion, compare the current TabMailAuthService.getSession()?.userId to the captured purchaser id; do not open the gate if they differ.

• This is a session-identity check, not the subscriptionOwnerUserId ownership check — the latter can be stale from a listener race on the purchase path and would risk false-closing a legitimate buyer, which is why Do not open the AI gate when restoring a different account's subscription #50 deliberately did not guard the purchase path with it.
• A legitimate buyer is the same user start→finish, so this check never false-closes a real purchase.
• Mirror the restore-gate ownership test shape from Do not open the AI gate when restoring a different account's subscription #50 (StoreKitManager Restore-Gate Ownership).

Acceptance

• Red-first test: pre-fix, the gate opens for the switched-to account.
• Green, two-sided: same-user completion opens; switched-user completion is withheld.
• Two-model vet on its own PR. Reuses the Do not open the AI gate when restoring a different account's subscription #50 ownership test pattern.

Why deferred

Degenerate and self-healing; no user-visible harm beyond a transient wrong-open that the next revalidation corrects. Tracked for purchase-path parity with #50, to be picked up when convenient. Related: #50 (restore-path guard).