WORKOS_COOKIE_DOMAIN env var ignored unless set via configure()

[DrunkOldDog]

Describe the bug
Setting WORKOS_COOKIE_DOMAIN has no effect in @workos/authkit-tanstack-react-start. The session cookie is written without a Domain attribute, so it can't be shared across subdomains. The equivalent env var works fine in @workos-inc/authkit-nextjs.

The only way to get the domain applied is to call configure({ cookieDomain: '...' }) from @workos/authkit-session manually before the service initializes.

To Reproduce
Steps to reproduce the behavior:

1. Start from the official tanstack example: https://github.com/TanStack/router/tree/main/examples/react/start-workos
2. Add WORKOS_COOKIE_DOMAIN=app.localhost (alongside the usual WORKOS_* vars) to .env.
3. Sign in and inspect the Set-Cookie response header for the wos-session cookie (DevTools → Network, or Application → Cookies).
4. The cookie is written with no Domain attribute, WORKOS_COOKIE_DOMAIN is ignored.

Expected behavior
WORKOS_COOKIE_DOMAIN should be read from the environment and applied as the Domain attribute of the session cookie, with parity to @workos-inc/authkit-nextjs, without needing a manual configure() call.

Desktop (please complete the following information):

• OS: macOS Tahoe 26.5.1
• Browser: chrome
• @workos/authkit-tanstack-react-start version: >=0.5.0 (also happening in latest 0.11.0)
• @workos/authkit-session version: >=0.3.4 (also happening in latest 0.7.1)
• Tanstack Start version: 1.168.27

Additional context
@workos-inc/authkit-nextjs reads WORKOS_COOKIE_DOMAIN directly through its own env-variables.ts, so it's unaffected — this is specific to the @workos/authkit-session ConfigurationProvider used by the TanStack (and other framework) adapters.