[BUG] toNexusAccount fails when the user has no wallet extensions installed

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Package Version

1.2.4

### Current Behavior

The library expects an Ethereum wallet extension installed, so if a user has no wallet extensions installed (`window.ethereum` === `undefined`), for example by connecting using a 3rd party connector via email or their web2 account, `toNexusAccount` fails with the following error:
```
TypeError: Cannot read properties of undefined (reading 'request')
at Object.transport (_esm-BSwfxQyV.js?v=4ff14d18:5862:21)
at createClient (parseSignature-BmMoHAM7.js?v=4ff14d18:25:48)
at createWalletClient (_esm-BSwfxQyV.js?v=4ff14d18:5816:9)
at toWalletClient (@biconomy_abstractjs.js?v=4ff14d18:70507:9)
at toNexusAccount (@biconomy_abstractjs.js?v=4ff14d18:70643:23)
```
```js
const walletClient = toWalletClient({
unresolvedSigner: _signer,
resolvedSigner: signer,
chain,
transport: transportConfig
});
```
```js
//#region node_modules/.pnpm/@biconomy+abstractjs@1.2.4_@metamask+delegation-toolkit@0.11.0_viem@2.52.2_bufferutil@4_2a944230c8f4986aa78e4c29ea8e997a/node_modules/@biconomy/abstractjs/dist/_esm/account/utils/toWalletClient.js
var toWalletClient = ({ unresolvedSigner, resolvedSigner, chain, transport }) => {
return createWalletClient(unresolvedSigner?.transport?.key === "custom" ? {
account: resolvedSigner.address,
chain,
transport: custom(window?.ethereum)
} : {
account: resolvedSigner,
chain,
transport
}).extend(publicActions);
};
//#endregion
```
```js
//#region node_modules/.pnpm/viem@2.52.2_bufferutil@4.1.0_typescript@6.0.3_utf-8-validate@6.0.6_zod@4.4.3/node_modules/viem/_esm/clients/createWalletClient.js
function createWalletClient(parameters) {
const { key = "wallet", name = "Wallet Client", transport } = parameters;
return createClient({
...parameters,
key,
name,
transport,
type: "walletClient"
}).extend(walletActions);
}
//#endregion
```
```js
//#region node_modules/.pnpm/viem@2.52.2_bufferutil@4.1.0_typescript@6.0.3_utf-8-validate@6.0.6_zod@4.4.3/node_modules/viem/_esm/clients/transports/custom.js
/**
* @description Creates a custom transport given an EIP-1193 compliant `request` attribute.
*/
function custom(provider, config = {}) {
const { key = "custom", methods, name = "Custom Provider", retryDelay } = config;
return ({ retryCount: defaultRetryCount }) => createTransport({
key,
methods,
name,
request: provider.request.bind(provider),
retryCount: config.retryCount ?? defaultRetryCount,
retryDelay,
type: "custom"
});
}
//#endregion
```

### Expected Behavior

_No response_

### Steps To Reproduce

Run a working app on incognito (no Ethereum wallet extensions enabled)

### Package.json (or lockfile) content

```json
{
"name": "xxx",
"private": true,
"version": "1.0.0",
"type": "module",
"scripts": { "xxx": "xxx" },
"dependencies": {
"@biconomy/abstractjs": "^1.2.4",
"@dynamic-labs/ethereum": "^4.88.6",
"@dynamic-labs/sdk-react-core": "^4.88.6",
"@dynamic-labs/wagmi-connector": "^4.88.6",
"@tailwindcss/vite": "^4.3.1",
"@tanstack/react-query": "^5.101.0",
"alchemy-sdk": "^3.6.5",
"daisyui": "^5.5.23",
"firebase": "^12.14.0",
"permissionless": "^0.3.6",
"react": "^19.2.7",
"react-dom": "^19.2.7",
"react-icons": "^5.6.0",
"react-router": "^7.17.0",
"react-toastify": "^11.1.0",
"tailwindcss": "^4.3.1",
"viem": "^2.52.2",
"wagmi": "^3.6.16"
},
"devDependencies": {
"@eslint/js": "^10.0.1",
"@types/node": "^24.13.2",
"@types/react": "^19.2.17",
"@types/react-dom": "^19.2.3",
"@vitejs/plugin-react": "^6.0.2",
"eslint": "^10.5.0",
"eslint-plugin-react-hooks": "^7.1.1",
"eslint-plugin-react-refresh": "^0.5.2",
"globals": "^17.6.0",
"typescript": "~6.0.3",
"typescript-eslint": "^8.61.0",
"vite": "^8.0.16"
}
}
```

### Link to Minimal Reproducible Example (StackBlitz, CodeSandbox, GitHub repo etc.)

_No response_

### Anything else?

My workaround now is by doing:
```ts
if (!window.ethereum) {
window.ethereum = {
async request(_: any): Promise {},
} as any;
}

toNexusAccount({
chainConfiguration: {
chain,
transport: http(import.meta.env.VITE_BASE_SEPOLIA_RPC_URL),
version: getMEEVersion(MEEVersion.V2_3_0),
versionCheck: false,
},
...
});
```
This works, but it would be great if abstractjs doesn't assume a wallet client / handle the case for `transport: custom(window?.ethereum)` when `window?.ethereum` is `undefined`

Thanks

Start at the toWalletClient call shown in the report and reproduce with no window.ethereum, using the supplied transport. Trace how toNexusAccount selects custom(window?.ethereum), then add coverage for the no-extension path and verify that toNexusAccount completes without reading request from undefined.

Written by the indexing model from the issue text.