fix(monetization): hold gifts with invalid recipients

[draxxptb]

Summary

• reject malformed persisted gift recipients before any paid delivery
• keep the receipt retryable and emit a diagnostic log code instead of coercing the recipient to userId 0
• cover the online and offline gift receipt paths with a regression test

Problem

A malformed pending gift intent without a RecipientId could reach the receipt path, where tonumber(intent.RecipientId) or 0 routed the paid gift to the DataStore key for userId 0 and returned PurchaseGranted.

Changes

• validate persisted recipient IDs as positive, finite integers
• return NotProcessedYet and log GIFT_INVALID_RECIPIENT when validation fails
• declare and document the new log code
• replace the defect characterization with a regression assertion

Validation

• lune run lune/run-tests
• SCRIBE_FRAGMENT_ALL=1 lune run lune/run-tests
• SCRIBE_SPECS=gifting/GiftIntentNet lune run lune/run-tests
• selene src test lune addons
• positive and negative Luau type-check fixtures
• log-code declaration/documentation check
• clock, thread, and random seam checks
• Stylua check on the committed Lua files

No related upstream issue.