Enabling `bundle: false` does not add import extensions in ESM files

I've noticed this bug after merging the following PR: https://github.com/TanStack/query/pull/5597

When `bundle: true` is set, `index.js` (ESM) file looks like this:

```
import "./chunk-55J6XMHW.js";
import {
QueriesObserver
} from "./chunk-FVYPMHO2.js";
import {
QueryClient
} from "./chunk-6F4EPMPD.js";
import {
QueryCache
} from "./chunk-5EQKN4WB.js";
import "./chunk-CSKZ3U2N.js";
...
```

When `bundle: false` is set, `index.js` (ESM) file looks like this:

```
import { CancelledError } from "./retryer";
import { QueryCache } from "./queryCache";
import { QueryClient } from "./queryClient";
import { QueryObserver } from "./queryObserver";
import { QueriesObserver } from "./queriesObserver";
import { InfiniteQueryObserver } from "./infiniteQueryObserver";
...
```

I expected that tsup would also add the required `.mjs` (or `.js` when `"type": "module"` is set) to relative imports within the project, like rollup did. This causes import errors when imported in ESM projects (e.g. Vite). For now, I've added https://github.com/favware/esbuild-plugin-file-path-extensions, but this seems to force override some tsup settings.

Reproduce the issue with tsup configured with bundle: false and inspect the generated index.js ESM imports, comparing them with the bundle: true output. Verify the fix by importing the generated package from an ESM project such as Vite and confirming relative imports resolve with the required extensions.

Written by the indexing model from the issue text.