Update deferred Stripe renewal and retry handling

[atavism]

• Handle both PaymentIntent and SetupIntent subscription flows
• Prevent PaymentSheet retries from creating duplicate subscriptions
• Adds Stripe tests covering both intent modes, retries, initialization, and error filtering

[coderabbitai]

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[Copilot]

Pull request overview

This PR updates the app’s deferred Stripe PaymentSheet flow to support both PaymentIntent and SetupIntent subscription confirmations (notably for renewal/trial scenarios), while preventing retries from creating duplicate backend subscriptions. It also adds widget tests to validate intent-mode selection, retry behavior, and user-facing Stripe error filtering.

Changes:

• Add StripeIntentMode + stripeIntentModeForRenewal(...) and use intent-mode–specific confirmation secrets in the deferred confirm handler.
• Cache the in-flight subscription creation request during PaymentSheet retries to avoid duplicate subscriptions.
• Add Stripe service widget tests for publishable-key initialization ordering, retry semantics, setup-mode confirmation, and error-message filtering.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
lib/core/services/stripe_service.dart	Adds intent-mode selection and retry-safe subscription creation for deferred PaymentSheet confirmation; updates Stripe error filtering.
lib/features/auth/choose_payment_method.dart	Selects the correct Stripe intent mode for renewals (PaymentIntent vs SetupIntent) and passes it into StripeService.
lib/lantern_app.dart	Makes the builder safer when child is null by providing a fallback widget.
test/core/services/stripe_service_test.dart	Introduces widget tests validating deferred Stripe flow behavior across intent modes, retries, and error filtering.

Suppressed comments (1)

lib/core/services/stripe_service.dart:196

• The empty-secret path throws a hard-coded English string that will be shown to users in the PaymentSheet retry UI. This bypasses the app’s i18n approach (used elsewhere in this file via .i18n) and makes the message non-localizable.

      if (secret.isEmpty) {
        throw Exception(
          'Please try again after some time. If the issue persists, contact support.',
        );

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[jigar-f]

LGTM, Thanks for adding test