feat: add WhatsApp non-custodial signer support

[maxsch-xmint]

Description

Adds support for WhatsApp non-custodial signers to the wallets SDK, mirroring the phone signer flow with a whatsapp:<phone> locator.

• New WhatsappSignerConfig, WhatsappInternalSignerConfig, and WhatsappSignerLocator types exported from signers/types.
• New whatsapp.ts signer descriptor and registration in getSignerDescriptor.
• EVMNonCustodialSigner, SolanaNonCustodialSigner, and StellarNonCustodialSigner constructors accept WhatsApp internal configs.
• getSignerLocator, extractSignerBase, Signer union, onAuthRequired callback, and wallet types updated for whatsapp.
• openapi.json updated with whatsapp signer schemas and api/gen regenerated.

Test plan

• pnpm --filter @crossmint/wallets-sdk test:vitest ✅
• pnpm lint ✅ (warnings only)

Package updates

Added changeset .changeset/whatsapp-signer.md for @crossmint/wallets-sdk (minor).

Link to Devin session: https://crossmint.devinenterprise.com/sessions/8701395d99694039a763477fe6b941ee
Requested by: @maxsch-xmint

[changeset-bot]

🦋 Changeset detected

Latest commit: 5553196

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@crossmint/wallets-sdk	Minor
@crossmint/wallets-quickstart-devkit	Patch
@crossmint/wallets-playground-react	Patch
@crossmint/client-sdk-react-base	Patch
@crossmint/client-sdk-react-native-ui	Patch
@crossmint/client-sdk-react-ui	Patch
@crossmint/wallets-playground-expo	Patch
@crossmint/auth-ssr-nextjs-demo	Patch
@crossmint/client-sdk-nextjs-starter	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[devin-ai-integration]

Original prompt from Max

We have finally created a Whatsapp signer in Twilio. Let's add support for a new type of non custodial signer, a whatsapp signer. For this you will have to edit @Paella-Labs/crossbit-main , @Crossmint/open-signer and @Crossmint/crossmint-sdk . On the open-signer, leave the environment variables there, I'll fill them up once I have them.
This should basically mimic the phone signer, but the OTP at the end will be sent by whatsapp instead of sms

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring