feat: add Stripe Identity Verification support

[realmeylisdev]

Summary

This PR adds support for Stripe Identity Verification, allowing Flutter apps to verify users' government-issued IDs and selfies using Stripe's native IdentityVerificationSheet.

Closes #1819

API Usage

final result = await Stripe.instance.presentIdentityVerificationSheet(
  verificationSessionId: 'vs_xxx',
  ephemeralKeySecret: 'ek_xxx',
  brandLogo: 'base64EncodedImage', // Optional, iOS only
);

switch (result) {
  case IdentityVerificationCompleted():
    print('Verification completed');
  case IdentityVerificationCanceled():
    print('User canceled');
  case IdentityVerificationFailed(:final error):
    print('Error: ${error.message}');
}

Changes

Platform Interface Layer

• Add IdentityVerificationSheetParams, IdentityVerificationResult (sealed class), and IdentityVerificationError models using Freezed
• Add presentIdentityVerificationSheet abstract method to StripePlatform
• Implement method channel communication in MethodChannelStripe

iOS Implementation

• Add StripeIdentity dependency to Package.swift
• Create StripeSdkImpl+Identity.swift extension with native implementation
• Support optional brand logo via base64 encoded image

Android Implementation

• Add com.stripe:identity dependency to build.gradle
• Create IdentityVerificationSheetManager following the StripeUIManager pattern
• Register method in StripeSdkModule and StripeAndroidPlugin

Web

• Add stub that throws WebUnsupportedError (Identity Verification requires native mobile SDKs)

Example App

• Add IdentityVerificationScreen with demo implementation

API Compatibility

Feature	Stripe iOS SDK	Stripe Android SDK	This Implementation
Method	IdentityVerificationSheet.present()	IdentityVerificationSheet.present()	✅ presentIdentityVerificationSheet()
verificationSessionId	✅	✅	✅
ephemeralKeySecret	✅	✅	✅
brandLogo	✅ (UIImage)	✅ (Uri)	✅ (base64, iOS only)
Result: Completed	.flowCompleted	Completed	✅ IdentityVerificationCompleted
Result: Canceled	.flowCanceled	Canceled	✅ IdentityVerificationCanceled
Result: Failed	.flowFailed(error)	Failed(throwable)	✅ IdentityVerificationFailed(error)

Note on React Native

The original issue mentions "React native supports it" - however, after investigation, React Native Stripe SDK appears to have verifyIdentity only as part of the Onramp feature, not as a standalone Identity Verification Sheet. This implementation provides the full native Identity Verification Sheet functionality that matches the official Stripe iOS and Android SDKs directly.

Test Plan

• iOS: Identity sheet presents correctly
• iOS: User can complete verification flow
• iOS: Cancellation returns correct result
• iOS: Errors are properly propagated
• Android: Identity sheet presents correctly
• Android: User can complete verification flow
• Android: Cancellation returns correct result
• Android: Errors are properly propagated
• Web: Throws appropriate unsupported error

Testing Requirements

• Requires a backend endpoint that creates a VerificationSession and returns {id, ephemeral_key_secret}
• Identity must be enabled in Stripe Dashboard
• Real device recommended for camera-based verification (not simulator)
• Camera permissions must be configured (NSCameraUsageDescription on iOS)

[realmeylisdev]

@remonh87 - This PR implements the Stripe Identity Verification feature requested in #1819.

Would you be able to review when you have a chance?

Note about CI: The 16 test failures in stripe_js are unrelated to this PR - they occur because GitHub Actions doesn't pass repository secrets (STRIPE_PUBLISHABLE_KEY) to PRs from forks. The same tests fail on other fork PRs (e.g., #2317, #2316) and pass when run from the main repo.

Thanks!

[jonasbark]

Thanks for this PR!

We currently synchronize our code base with the one from Stripe React Native. Most of your Android / iOS changes would be overwritten then.

There are a few possible ways to go ahead:

1. create an extra package similar to https://github.com/stripe/stripe-identity-react-native
2. or move the changes from the synchronized classes to e.g. an extension function of their respective classes

Since it introduces another library dependency, I would opt for variant 1)

[realmeylisdev]

Thanks for the guidance @jonasbark!

I've created a new PR #2319 with the separate package approach following the stripe-identity-react-native pattern.

The new structure keeps Identity completely isolated from the main package:

• Separate method channel (flutter.stripe_identity/identity)
• Standalone native plugins (won't be affected by React Native sync)
• Independent versioning

Closing this PR in favor of #2319.