fix(ios): report StoreKit 2 introductory/trial price, not full price

[NickSxti]

Summary

Fixes SUP3-154. On iOS, the StoreKit 2 purchase flow reported the introductory/trial price as the full subscription price, overstating first-period revenue. StoreKit 1 was unaffected.

QNRequestSerializer.purchaseInfo:receipt: (the SK2 path) set introductory_offer.value = purchaseModel.price (the standard price) instead of purchaseModel.introductoryPrice. The model already carried the correct value from PurchasesMapper (StoreKit's introductoryOffer.price); the serializer simply read the wrong field.

While fixing this, the sibling promo_offer block in the same method turned out to have the same spurious-block defect, so it is corrected in a follow-up commit.

Fix

Introductory offer

Guard the introductory_offer block on introductoryPrice != nil and read introductoryPrice for the value. One change fixes two defects:

• Wrong value: the intro/trial price is now reported correctly (free trial -> "0", paid intro -> discounted price).
• Spurious block: the old introOffer.count > 0 guard was always true because price is nonnull, so an introductory_offer was emitted even for purchases with no offer. The new guard omits it.

Promotional offer

The SK2 promo_offer block was built and attached unconditionally, so an empty promo_offer {} was emitted on every purchase that had no promotional offer - the same spurious-block pattern. Guard it on promoOfferId != nil.

Both fixes mirror the existing SK1 path (purchaseData:..., which guards introductory_offer on product.introductoryPrice != nil and promo_offer on the purchased offer id) and PurchasesMapper (which populates the introductory_* / promoOffer_* fields only when an offer exists). SK1 and SK2 send to the same purchase endpoint, which already tolerates an absent introductory_offer / promo_offer because the SK1 path omits them - so real offer purchases are unchanged on the wire and only the empty/phantom blocks are dropped.

Tests

Adds regression coverage for the previously-untested SK2 purchaseInfo:receipt: path in QRequestSerializerTests.m:

• paid intro: every introductory_offer field is asserted; value equals the discounted price and not the full price
• free trial: value equals "0" (not dropped)
• no offer: introductory_offer is omitted
• promo present: every promo_offer field is reported
• no promo: promo_offer is omitted

Verified with xcodebuild test -only-testing:QonversionTests/QNRequestSerializerTests (6/6 passing).

Notes

• Analytics/reporting only; entitlements and Apple billing are unaffected.
• Historical SK2 rows remain wrong and would need an App Store Server API backfill (the original payloads carry the wrong value) - out of scope here.

[NickSxti]

Behavioral note: this guard also changes the no-offer case. Previously an introductory_offer block was emitted on every purchase here, because the old introOffer.count > 0 check was always true (the value was set from price, which is nonnull). With this guard, purchases without an introductory offer omit the block entirely.

That matches the StoreKit 1 path above (purchaseData: guards on product.introductoryPrice != nil), so it should be safe - just flagging that the request body shape changes for no-offer StoreKit 2 purchases, in case anything consuming the payload expects the block to always be present.

[NickSxti]

Minor, and out of scope for this fix: the promo_offer block is still built and assigned unconditionally (result[@"promo_offer"] = [promoOffer copy] a few lines down), so a purchase with no promotional offer sends an empty promo_offer: {}. That is the same always-emitted-block pattern this PR just fixed for introductory_offer.

The StoreKit 1 path guards the equivalent block on offerId.length > 0. Might be worth a follow-up to guard this on promoOfferId != nil for parity.