send idempotency keys with redirect checkout

[atavism]

For https://github.com/getlantern/engineering/issues/3434 and https://github.com/getlantern/engineering/issues/3292

[Copilot]

Pull request overview

This PR threads an idempotencyKey through the app’s payment redirect flows (Flutter → platform channels/FFI → Go core) so the backend can make redirect-checkout/session creation idempotent, addressing the linked engineering issues.

Changes:

• Added idempotencyKey as a required parameter to the Dart payment redirect APIs and passed it through both FFI and platform-channel implementations.
• Generated an idempotency key in PaymentNotifier and included it in redirect calls.
• Updated Go mobile/FFI exports and core payment methods to accept and forward the idempotency key in the redirect request body.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
macos/Runner/Handlers/MethodHandler.swift	Reads idempotencyKey from method-channel args and passes it into Go mobile payment redirect calls.
android/app/src/main/kotlin/org/getlantern/lantern/handler/MethodHandler.kt	Passes idempotencyKey from method-channel args into Mobile.paymentRedirect.
lib/lantern/lantern_core_service.dart	Extends core service interface with required idempotencyKey for redirect methods.
lib/lantern/lantern_service.dart	Propagates idempotencyKey through the top-level service wrapper.
lib/lantern/lantern_platform_service.dart	Sends idempotencyKey via method channel for macOS/Android redirect flows.
lib/lantern/lantern_ffi_service.dart	Sends idempotencyKey into FFI calls for redirect flows.
lib/lantern/lantern_generated_bindings.dart	Updates generated FFI bindings signatures to include idempotencyKey.
lib/features/plans/provider/payment_notifier.dart	Generates and supplies an idempotency key when starting redirect flows.
lib/core/common/common.dart	Adds generatePaymentRedirectIdempotencyKey() helper.
lantern-core/core.go	Adds idempotencyKey to redirect APIs and forwards into account.PaymentRedirectData.
lantern-core/ffi/ffi.go	Updates exported C functions to accept and forward idempotencyKey.
lantern-core/mobile/mobile.go	Updates gomobile exports to accept and forward idempotencyKey.

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[jigar-f]

@atavism Do you think we should just create the idempotencyKey key here, since generation is basically the same, so we don't have to pass it through all the platforms? Is there any issue with that?