Using duplex() with fd3: it must be a writable stream
I am trying to use `execa` within a `pipeline`, which results in very elegant code.
However, I am facing a problem when using an input file descriptor different from stdin. The following snippet contains a
<details><summary>minimal reproducer</summary>
<p>
```js
// Importing dependencies
import fs from 'node:fs';
import dedent from 'dedent';
import { execa } from 'execa';
import { pipeline } from 'node:stream/promises';
import { Readable } from 'node:stream';
// Run execa as duplex stream in a pipeline
// The pipeline source and sink, as well as the subprocess (Python here) are not important,
// instead the focus is on the "to" option for duplex()
await pipeline(
Readable.from('This is a test'),
execa(
'python3',
{
input: dedent(`
import os
with os.fdopen(3, 'r') as file:
content = json.load(file)
print(content)
`),
stdio: ['pipe', 'pipe', 'pipe', 'pipe'],
},
).duplex({ to: 'fd3' }),
process.stdout,
);
```
</p>
</details>
Running the code on GNU/Linux 6.14.0 with Node.js v22.15.0 and execa 9.6.0 causes the following
<details><summary>error message</summary>
<p>
```
file:///tmp/test/node_modules/execa/lib/arguments/fd-options.js:66
throw new TypeError(`"${getOptionName(isWritable)}" must not be ${fdName}. It must be a writable stream, not readable.`);
^
TypeError: "to" must not be fd3. It must be a writable stream, not readable.
at validateFdNumber (file:///tmp/test/node_modules/execa/lib/arguments/fd-options.js:66:9)
at getFdNumber (file:///tmp/test/node_modules/execa/lib/arguments/fd-options.js:36:2)
at getToStream (file:///tmp/test/node_modules/execa/lib/arguments/fd-options.js:7:19)
at getSubprocessStdin (file:///tmp/test/node_modules/execa/lib/convert/writable.js:32:26)
at createDuplex (file:///tmp/test/node_modules/execa/lib/convert/duplex.js:22:68)
at main (file:///tmp/test/[eval1]:29:11)
at file:///tmp/test/[eval1]:37:2
at ModuleJob.run (node:internal/modules/esm/module_job:274:25)
at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:268:26)
at async ModuleLoader.executeModuleJob (node:internal/modules/esm/loader:264:20)
```
</p>
</details>
I tracked the source, and noticed that by simply removing the [validation rules](https://github.com/sindresorhus/execa/blob/103095f2c9a459ae3500fa7d861f162c21a94b3a/lib/arguments/fd-options.js#L61-L67), the code runs and behaves as expected. Unfortunately, I am not familiar enough with the source code to suggest a solution, so I would appreciate any help by the maintainers or more experienced users.
Thank you for a wonderful library, and its extensive documentation.
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