feat(api): Add organization authentication config endpoint (#122277)

Adds a private, unauthenticated organization authentication-config
endpoint for the React login flow. The response describes the
organization, password or SSO login method, provider metadata, whether
SSO is required, registration and join-request state, caller
authentication state, and relevant warnings without establishing a new
session. SSO remains optional when the provider allows unlinked members,
and the endpoint is rate-limited by IP.

`authenticated` reports the caller’s global Sentry session, while
`userIsAuthenticated` additionally requires usable access to the
requested organization, including completed SSO where required. A
persisted configuration whose provider is no longer registered returns a
generic temporary-unavailability response rather than offering an
invalid password fallback.