CryptoOnramp SDK: Remove authenticateUser API

[mliberatore]

Summary

Removes the authenticateUser API, as well as its associated return type, and other references to it. After this is merged, it will also be removed from the API review document (referenced below).

Motivation

Comments and questions on 2026-01-13 - CryptoOnramp SDK (Mobile API Review) Gated GA have led to an understanding that this API is not needed, that OAuth scopes will be required to authenticate (which is possible via the authorize API).

Testing

The code in question was not in use in the codebase, so to test, I simply built the project and ensured tests passed.

Changelog

It sounds like we'll need to provide some detail for release, since we're removing an API.

Sorry, before merging could we update the CHANGELOG with a note of this deprecation?

[mats-stripe]

I think we could add something like this to CHANGELOG.md:

### CryptoOnramp (Beta)
* [Removed] Removed `CryptoOnrampCoordinator.authenticateUser()` in favor of `CryptoOnrampCoordinator.authenticateUserWithToken()`.

[mliberatore]

I think we could add something like this to CHANGELOG.md:

### CryptoOnramp (Beta)
* [Removed] Removed `CryptoOnrampCoordinator.authenticateUser()` in favor of `CryptoOnrampCoordinator.authenticateUserWithToken()`.

Oops, updated CHANGELOG before I saw your reply. I’ll tweak it to match yours exactly.

[mliberatore]

I think we could add something like this to CHANGELOG.md:

### CryptoOnramp (Beta)
* [Removed] Removed `CryptoOnrampCoordinator.authenticateUser()` in favor of `CryptoOnrampCoordinator.authenticateUserWithToken()`.

Oops, updated CHANGELOG before I saw your reply. I’ll tweak it to match yours exactly.

Actually, the "replacement" API should be authorize, not the seamless sign-in API. I’ll add the "(Beta)" designation from your suggestion, though.

[mliberatore]

Note that this is in the "pending release" section. LMK if I should move it to today's release, but note that there's no rush for me to merge this before the release is made.

[mats-stripe]

LGTM! I think it's too late for today's release

[mats-stripe]

LGTM! I think it's too late for today's release

[github-actions]

✅ Dead code has been resolved in this PR.

[find-dead-code]

[mliberatore]

Merge conflict resolution in CHANGELOG.md triggered a review dismissal. Would you mind re-approving @mats-stripe?