Render App Bridge instead of login redirect when shop/host params are missing in embedded apps

[byrichardpowell]

Problem

During local development of embedded Shopify apps, there's an annoying workflow where the login screen appears inappropriately:

1. Navigate to a new URL within the app (the id_token / shop / host query params are lost from the URL)
2. Make a code change
3. Vite triggers a full page reload (this happens intermittently — not all changes trigger HMR, some cause a full reload)
4. The server receives a document request with no shop or host params and no Authorization header
5. validateShopAndHostParams redirects to the login page

This is incorrect because the app is still running inside the Shopify admin iframe — it just lost the URL search params during SPA navigation + reload.

Solution

In validateShopAndHostParams, when shop or host params are missing/invalid, we now render a minimal App Bridge page instead of redirecting to login.

When App Bridge loads inside the Shopify admin iframe, it:

1. Detects it's embedded in the admin
2. Retrieves the session token from the parent frame
3. Re-authenticates the app seamlessly

This is the same mechanism used by the existing "bounce page" flow (/auth/session-token), just applied earlier in the auth pipeline.

The previous code had a throw redirect(config.auth.loginPath) fallback for non-embedded apps, but isEmbeddedApp is hardcoded to true in shopify-app.ts for all apps using this library (the ShopifyAdmin distribution is already excluded earlier in the function). This made the login redirect dead code. This PR removes that dead code — no public APIs are changed.

Note: this is a production behavior change, not just a dev improvement. Any request to an embedded app without shop/host params (e.g., bookmarked deep links, direct URL access) will now get the App Bridge page instead of a login redirect. This is correct behavior — App Bridge will handle re-embedding in the admin.

What changed

• validate-shop-and-host-params.ts: Replaced the redirectToLoginPath helper with renderAppBridgeOrError. Instead of redirecting to the login page, renders the App Bridge page so it can recover the session. Removed the unused redirect import from react-router and the dead login redirect code path. The login path guard (500 error when authenticate.admin() is called from the login route) is preserved.
• doc-request-path.test.ts: Updated the 4 test cases for missing/invalid shop/host params to expect an App Bridge page (200 with App Bridge script tag) instead of a login redirect (302).
• reject-bot-request.test.ts: Updated the "allowed Shopify agents" assertions to expect 200 (App Bridge page) instead of 302 (login redirect), confirming the request passes the bot check and proceeds to auth validation.
• expect-login-redirect.ts: Removed — no longer used by any test.
• __test-helpers/index.ts: Removed the expect-login-redirect export.