fix(plugin-multi-tenant): tenant selector not appearing after login (#15617)

When a custom provider in `admin.components.providers` changes its
wrapper component type on login (e.g. `<Context.Provider>` →
`<Fragment>`), React remounts `TenantSelectionProviderClient`. On
remount, `useState(initialTenantOptions)` gets stale props and
`useRef(userID)` re-initializes so `syncTenants()` never fires, leaving
the tenant selector empty. This only affects projects with a custom
provider that conditionally changes its tree structure based on auth
state - default setups are unaffected since no built-in provider does
this.

The fix adds a `useEffect` that syncs `initialTenantOptions` from the
server component into client state whenever the prop changes, making it
resilient to remounts. Also fixes `test/dev.ts` not forwarding config
overrides to import map generation.