fix: do not offer sign-up on an instance whose sign-up mode does not allow it

[dbeattie71]

The problem

ClerkAuthentication renders the "Don't have an account? Sign up" toggle unconditionally, so on
a restricted or waitlist instance it offers an account it cannot create. Filling in the form it
leads to ends in the API refusing the sign-up — an affordance whose only possible outcome is a
rejection.

Observed on a production instance whose access mode is Invite-only: the signed-out screen
renders a full sign-up form, First name through Password, for an instance that accepts none of it.

The mode is already parsed — nothing consulted it

user_settings.sign_up.mode is read into SignUpSettings.mode and reachable as
Auth.env.user.signUp.mode. This adds Environment.signUpIsOpen beside hasOauthStrategies, and
_ClerkAuthenticationState.build already reads authState.env for the OAuth panel — so the widget
needs no new dependency to answer the question.

bottomPortion: authState.env.signUpIsOpen ? _BottomPortion(...) : emptyWidget,

Two deliberate choices

Only public reads as open. restricted requires an invitation and waitlist collects an
address for later; the SDK implements neither flow, so on those instances the form cannot succeed.
An unrecognised value is far likelier to be a mode this getter has not heard of than a reason to
offer an account to everyone — so it fails closed.

Only the toggle is gated, never the panel. If something has already put the auth state into
signing-up, ClerkSignUpPanel still renders. A sign-up in flight must not become a blank card, and
hiding a form somebody is part-way through would be a worse failure than showing one that is
refused. _state starts at signingIn and, with no toggle, only the existing coercion from
authState.isSigningUp can move it — so nothing that works today stops working.

Tests

Three cases added to environment_test.dart, following the hasOauthStrategies group: public is
open; restricted and waitlist are not; an unknown mode, an empty mode and Environment.empty
are not. dart test green on clerk_auth, flutter analyze clean on clerk_flutter (one
pre-existing unnecessary_import info in an unrelated test file).

Alternative considered

A signInOnly parameter on the widget. Rejected: the widget's whole design is to take no
parameters and read the instance instead, and the instance already states this. A caller should not
have to restate what Clerk has already said.