fix: Error resulting from hard cast from readable to writable

[elliott-with-the-longest-name-on-github]

This is just another example of why as X is evil and shouldn't ever be used. A Readable was being cast to a Writable.

I stuck a button in my test app with

	<button
		onclick={() => {
			console.log('adding tool result');
			addToolResult({
				toolCallId: 'call_wLJFuAcFfygtlxd3nAtYRpt8',
				result: { location: 'Denver, CO', temperature: 68 }
			});
		}}>Add Tool Result</button
	>

on main this causes the failure reported in the #5056 comments. With this branch, the error is gone and everything seems to be working correctly.

[wookiehangover]

thanks for the fix on this!