IAP SDK emits purchase events with empty store_transaction_id → verification never runs (regression in 5.4.1)

Android purchase-event - Affects: 5.4.1, 5.4.2
Summary
On Android, the SDK emits PurchasePaidEvent/PurchaseFulfilledEvent with an empty store_transaction_id (Google Play purchase token) and empty receipt. The verifier can't run — no protocol is selected — and returns IAP_RECEIPT_MISSING. The purchase is never verified.

• Impact*

• 3.8% of all verification events (5.7% of Android) over 2026-07-12 → 09-02.
• Version-correlated regression: 5.4.0 0.6% → 5.4.1 8.4% → 5.4.2 4.9%. Introduced in 5.4.1.
• Small iOS analogue: a handful of events emit placeholder '0'/'1' store_txn ids on first purchase → INVALID_RECEIPT. Same root class.

• Expected*
  SDK must not emit a paid/fulfilled purchase event without a valid store transaction id + receipt/token; hold and retry, or drop with an error, rather than emitting empties/placeholders.

Acceptance

• No purchase events with empty or placeholder (0/1) store_transaction_id.
• IAP_RECEIPT_MISSING rate on ≥ next SDK returns toward the 5.4.0 baseline (<1%).