feat(react-router): support request-scoped CSP nonces in createSentryHandleRequest

### Problem

`createSentryHandleRequest()` does not currently support a request-scoped Content Security Policy nonce.

React Router requires the same nonce in both places when rendering a Framework Mode document:

1. The `nonce` prop on ``
2. The `nonce` option passed to `renderToPipeableStream`

https://reactrouter.com/how-to/security

The current helper owns both calls but does not expose a nonce option, and its `loadContext` argument is unused:

https://github.com/getsentry/sentry-javascript/blob/10.70.0/packages/react-router/src/server/createSentryHandleRequest.tsx

A minimal setup generates a unique nonce in server middleware and stores it in React Router's request context:

```ts
const loadContext = new RouterContextProvider();
loadContext.set(cspContext, {
nonce: crypto.randomBytes(16).toString("base64"),
});
```

There is currently no way to pass that value through `createSentryHandleRequest()`. Applications must copy React Router's Node handler and manually compose `getMetaTagTransformer()` and `wrapSentryHandleRequest()`.

A static nonce option would not be sufficient because CSP nonces must be fresh and unpredictable for each rendered response.

### Suggested direction

Could the helper expose a request-aware mechanism for resolving the nonce? For example, without prescribing the final API:

```ts
getNonce({ request, loadContext }): string | undefined
```

The returned value would need to be passed to both:

```tsx

```

and:

```ts
renderToPipeableStream(element, { nonce, ... })
```

The helper should continue to provide Sentry's trace meta-tag injection and request wrapping.

Importing `@sentry/react-router/cloudflare` is not an appropriate workaround for a Node deployment. This application runs on Node 24 in AWS Lambda, uses `renderToPipeableStream` and Node streams, and does not run on Cloudflare. Additionally, the Cloudflare transformer cannot provide the nonce to ``.

### Versions tested

- `@sentry/react-router`: 10.70.0
- `react-router`: 8.3.0
- `@react-router/node`: 8.3.0
- `react`: 19.2.6
- `react-dom`: 19.2.6
- Node.js 24
- AWS Lambda

Start with packages/react-router/src/server/createSentryHandleRequest.tsx and the React Router security guidance linked in the issue. Trace how loadContext, ServerRouter, renderToPipeableStream, getMetaTagTransformer(), and wrapSentryHandleRequest() are used. Done means a request-scoped nonce can reach both rendering locations while Sentry trace meta-tag injection and request wrapping continue to work.

索引モデルが issue の本文から書いたものです。