Adapty SDK 4.1.0: contract 4.1 and the 4.1 natives on both platforms

[yauch-dev]

Moves the SDK to cross-platform contract 4.1.0 and the 4.1.0 natives on both platforms:
AdaptySDK-iOS 4.1.0 and, since the Android 4.1.0 release landed, crossplatform 4.1.0 /
android-sdk 4.1.0 / android-ui 4.1.0 with the bundled unity-wrapper AAR rebuilt at 4.1.0. The
version is 4.1.0 — the iOS-only snapshot this PR started as, and the not-for-publishing gate it
carried, are gone with the condition that raised them.

Breaking

• UpdateAttribution → UpdateExternalAttribution (source → provider),
  AdaptyProfile.AppliedAttributionSources → AppliedExternalAttributionProviders — matching the
  native 4.1 rename, no deprecated aliases. Wire key of the profile member unchanged.
• IAdaptyEventListener gained OnReceivePromotedPurchase(AdaptyPromotedProduct) — a deliberate
  abstract add, no default implementation: the iOS default is silently purchase, which should not
  hide in a default.
• Adapty Attribution is opt-in, and installation details come with it. The 4.1 natives collect
  installation details only when the configuration asks; the 4.0 natives collected them
  unconditionally. So OnInstallationDetailsSuccess stops firing and
  GetCurrentInstallationStatus stops reporting Determined until the app sets
  SetAdaptyAttributionEnabled(true) — on both platforms. It has its own migration step.
• The 4.1 natives read fallback file format 11 and reject format 10 at SetFallback, on both
  platforms: DecodingFailed (adapty_code: 2006) on iOS, WrongParam (adapty_code: 3001) on
  Android. Both demo stubs are format 11 and the guide tells users to re-download both files.

Added

• Adapty.MakePromotedPurchase + AdaptyPromotedProduct (App Store promoted purchases, iOS only).
  Note: the pinned native completes promoted purchases itself and does not emit the event yet
  (AdaptyPluginDelegate keeps the default implementation behind a TODO at tag 4.1.0); the listener
  method starts firing only once a future SDK release moves the exact pin to a native that reports
  them.
• AdaptyConfiguration.Builder.AdaptyAttributionEnabled (+SetAdaptyAttributionEnabled), off and
  unsent by default — see the breaking entry above for what that now costs.
• A product request writes its offer identifier nested as subscription.offer.offer_identifier
  through AdaptySubscriptionOfferRequest, shared by the paywall and promoted DTOs; the flat key is
  no longer written (natives read both, the contract documents only the nested form).
• AdaptyFlow carries ui_schema (UIBuilder 5.1 custom layouts) to the renderer as an opaque
  private JObject — round-trip only, not public API.
• MIGRATION-v4.0-to-v4.1.md, README pins and the changelog section (both copies, entry-set in
  sync).

Re-checked against the 4.1.0 natives, no change needed

• The error code set is the same on both platforms: iOS AdaptyError.swift is byte-identical
  between 4.0.2 and 4.1.0, and Android only added a @Deprecated annotation to 1004. The one
  behavioural change is that Android 4.1 no longer surfaces NoPurchasesToRestore —
  RestorePurchases completes with the current profile — and iOS never sent it, so the member is
  documented as unreachable at these pins instead of as Android-only.
• obfuscated_profile_id is still declared in the contract and implemented by nobody, iOS included,
  which is what AGENTS asks to confirm on every native bump.
• preloadFlows / preloadOnboardings and the customLayoutId argument that Android 4.1 added are
  not in the cross-platform contract, so there is nothing for this SDK to expose.
• Android reads the offer of a purchase request out of the base64 payload_data blob and drops the
  readable offer node, so writing the offer identifier nested only is safe there.

Deliberately not in this PR

• The iOS pin stays at 4.1.0. release/4.1.1 exists upstream (one commit — the visionOS build
  restore — and the version bump) but is not tagged or released, and an exact SwiftPM pin needs the
  tag. Moving to 4.1.1 is a follow-up once iOS ships it; it changes no API this SDK touches.
• The contract's offer-identifier union not listing type: "code" while both iOS and this SDK
  handle it — pre-existing, a question for the contract owners.

Testing

• JSON layer: full four-leg matrix green in CI (editor + iOS + Android + iOS/KidsMode).
• Contract conformance pass run against 4.1.0 (extractor mapping updated); it caught the missing
  #if UNITY_IOS gate on the promoted product's is_family_shareable, now aligned with the paywall
  product. The final commit changes no wire format, so the pass was not re-run for it.
• Simulator acceptance on iPhone 17 Pro against live backend with a StoreKit configuration:
  activation (fallback format 11 accepted), GetFlow calm10 + products, a purchase through the new
  nested offer envelope (intro offer applied, [Environment: Xcode]), profile update arriving via
  did_load_latest_profile (Premium: true), UpdateExternalAttribution, and full-screen flow
  rendering by native UIBuilder 5.1.
• Android device acceptance is still owed — the 4.1.0 natives here have not been run on a
  device. It is on the release-branch checklist: activation with the format 11 stub, GetFlow +
  products, a purchase, UpdateExternalAttribution, and installation details behind the new flag.