@metamask/connect-evm v0.9.0 uses browser transport (window.postMessage) on React Native instead of WebSocket relay

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## Bug Description

`@metamask/connect-evm` v0.9.0 does not work on React Native because it resolves to the browser transport (`windowPostMessageTransport`) instead of the React Native WebSocket transport.

## Root Cause

The package resolution chain breaks on React Native:

1. `@metamask/connect-evm` has **no `react-native` export** in `package.json`:
```json
// connect-evm/package.json exports
".": {
"import": "...",
"require": "...",
"default": "..."
// ❌ No "react-native" condition
}
```

2. `@metamask/multichain-api-client` also has **no `react-native` export**:
```json
// multichain-api-client/package.json exports
".": {
"import": "...",
"require": "..."
// ❌ No "react-native" condition
}
```

3. `@metamask/connect-multichain` **does** have a `react-native` export with `WebSocketTransport` (using `wss://mm-sdk-relay.api.cx.metamask.io`), but it's never reached because:

4. The `DefaultTransport` class in `connect-multichain` imports `getDefaultTransport()` from `@metamask/multichain-api-client`, which always returns `windowPostMessageTransport` — a browser-only transport using `window.postMessage()`.

## Result

On React Native:
- `client.connect()` hangs indefinitely — `window.postMessage()` is a no-op
- `window.addEventListener('message', ...)` never receives responses
- MetaMask deep link is never triggered (the SDK never reaches that code path)
- No error is thrown — the connection just silently hangs

## Expected Behavior

`createEVMClient()` should use the `WebSocketTransport` (Centrifuge relay at `wss://mm-sdk-relay.api.cx.metamask.io`) when running on React Native, as shown in the [React Native quickstart docs](https://docs.metamask.io/metamask-connect/evm/quickstart/react-native/).

## Environment

- `@metamask/connect-evm`: 0.9.0
- `@metamask/connect-multichain`: 0.11.0 (bundled)
- `@metamask/multichain-api-client`: 0.3.0
- `@metamask/mobile-wallet-protocol-core`: 0.4.0
- React Native: 0.81.5
- Expo: 54
- Metro bundler
- Android (Samsung Galaxy)

## Suggested Fix

Add `"react-native"` condition to the `exports` field in both:
1. `@metamask/connect-evm/package.json`
2. `@metamask/multichain-api-client/package.json`

Or ensure `DefaultTransport` in `connect-multichain` checks the platform and uses `WebSocketTransport` when `window.postMessage` is not available (i.e., React Native environment).

## Reproduction

```bash
npx expo init test-app
cd test-app
npm install @metamask/connect-evm react-native-get-random-values readable-stream
npx expo prebuild
```

```typescript
import { createEVMClient } from '@metamask/connect-evm';

const client = await createEVMClient({
dapp: { name: 'Test', url: 'https://test.com' },
api: { supportedNetworks: { '0x1': 'https://eth.llamarpc.com' } },
mobile: { preferredOpenLink: (url) => Linking.openURL(url) },
});

// This hangs forever — never opens MetaMask
await client.connect({ chainIds: ['0x1'] });
```

Inspect the package.json exports for @metamask/connect-evm and @metamask/multichain-api-client, then trace DefaultTransport in connect-multichain and its getDefaultTransport import. Reproduce the Expo/React Native example and verify that createEVMClient().connect() uses the WebSocket relay, opens the MetaMask deep link, and no longer hangs.

Written by the indexing model from the issue text.