fix(studio): keep organization_slug in oauth signup redirect (#48352)

Email signups inside a partner OAuth flow lose `organization_slug` on
the post-confirmation redirect: the OAuth branch in `SignUpForm`
hand-builds the `/authorize` return URL from only `auth_id` and `token`,
and the component's nuqs hook never reads the param at all. The consent
screen uses `organization_slug` to preselect and lock the partner's
requested org, so affected multi-org users land on an empty picker and
the partner's requested org is silently dropped. The GitHub-OAuth signup
path goes through `buildPathWithParams` and preserves the param, which
is how this went unnoticed.

I validated the drop in production traffic before fixing: joining
sign-up pageviews to their post-signup `/authorize` return on the
`auth_id` URL param (30d), 28 of 36 resolvable flows came back without
the slug, and the 8 that kept it were the GitHub branch.

## To test

Needs a partner OAuth authorize link that includes an org, opened
signed-out: `/dashboard/authorize?auth_id=<id>&organization_slug=<slug>`
(note `auth_id` records expire quickly, so generate a fresh authorize
request from an OAuth app).

- [x] Sign up with email from that flow; after confirmation the redirect
lands on `/authorize` with `organization_slug` still in the URL
- [ ] Consent screen shows the requested org preselected and locked
- [x] Same flow without `organization_slug` behaves as before (no
trailing empty params in the redirect URL)

## Linear

- fixes GROWTH-1031




<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved sign-up redirects during authentication flows by preserving
invitation tokens and organization information.
  * Enhanced handling of sign-up links containing organization details.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/studio/components/interfaces/SignIn/SignUpForm.tsx

@@ -67,6 +67,7 @@ export const SignUpForm = () => {
   const [searchParams] = useQueryStates({
     auth_id: parseAsString.withDefault(''),
     token: parseAsString.withDefault(''),
+    organization_slug: parseAsString.withDefault(''),
   })
 
   const trackFunnelError = useTrackFunnelError()
@@ -102,7 +103,12 @@ export const SignUpForm = () => {
     let redirectTo: string
 
     if (isInsideOAuthFlow) {
-      redirectTo = `${redirectUrlBase}/authorize?auth_id=${searchParams.auth_id}${searchParams.token && `&token=${searchParams.token}`}`
+      const authorizeParams = new URLSearchParams({ auth_id: searchParams.auth_id })
+      if (searchParams.token) authorizeParams.set('token', searchParams.token)
+      if (searchParams.organization_slug) {
+        authorizeParams.set('organization_slug', searchParams.organization_slug)
+      }
+      redirectTo = `${redirectUrlBase}/authorize?${authorizeParams.toString()}`
     } else {
       // Use getRedirectToPath to handle redirect_to parameter and other query params
       const { returnTo } = router.query