Attempting to path a separate Studio will cause Astro to crash with uninformative message

[narration-sd]

Describe the bug

The current code assumes use of its embedded Studio ability, and will cause a seeming Astro error if a fully pathed Studio url is attempted. However, after discussion with them, it's clear the problem is that @sanity/astro is in this case attempting to add an Astro route which is not within the Astro app, which it properly rejects.

This report will be referenced by a simple PR which avoids this problem, allowing separate Studios to be used.

To Reproduce

1. in a Sanity Astro project's astro.config, set studioBasePath to any full protocol-domain url (e.g. http://localhost:3333), rather than the usual path only (e.g. /admin)
2. attempt to start the app in dev
3. Note a crash occurs, with a spurious-seeming error from Astro

Expected behavior

A full protocol-domain url studioBasePath should be always accepted, and operate as expected if it points to an available Sanity Studio.

Which versions of Sanity are you using?

$ sanity versions
@sanity/cli (global) 3.53.0 (latest: 3.54.0)
@sanity/prettier-config 1.0.2 (up to date)

What operating system are you using?
Windows 11 very latest

Which versions of Node.js / npm are you running?

$ pnpm -v && node -v
9.1.2
v20.11.1

Additional context

@sanity/astro@3.1.4

Security issue?

none