🔒 fix: Request interceptor for Shared Link Page Scenarios (#12036)

* ♻️ refactor: Centralize `buildLoginRedirectUrl` in data-provider

Move `buildLoginRedirectUrl` from `client/src/utils/redirect.ts` into
`packages/data-provider/src/api-endpoints.ts` so the axios 401
interceptor (and any other data-provider consumer) can use the canonical
implementation with the LOGIN_PATH_RE guard and BASE_URL awareness.

The client module now re-exports from `librechat-data-provider`, keeping
all existing imports working unchanged.

* 🔒 fix: Shared link 401 interceptor bypass and redirect loop (#12033)

Fixes three issues in the axios 401 response interceptor that prevented
private shared links (ALLOW_SHARED_LINKS_PUBLIC=false) from working:

1. `window.location.href.includes('share/')` matched the full URL
   (including query params and hash), causing false positives. Changed
   to `window.location.pathname.startsWith('/share/')`.

2. When token refresh returned no token on a share page, the
   interceptor logged and fell through without redirecting, causing an
   infinite retry loop via React Query. Now redirects to login using
   `buildLoginRedirectUrl()` which preserves the share URL for
   post-login navigation.

3. `processQueue` was never called in the no-token branch, leaving
   queued requests with dangling promise callbacks. Added
   `processQueue(error, null)` before the redirect.

* ✅ test: Comprehensive 401 interceptor tests for shared link auth flow

Rewrite interceptor test suite to cover all shared link auth scenarios:

- Unauthenticated user on share page with failed refresh → redirect
- Authenticated user on share page with failed refresh → redirect
- share/ in query params does NOT bypass the auth header guard
- Login path guard: redirect to plain /login (no redirect_to loop)
- Refresh success: assert exact call count (toBe(3) vs toBeGreaterThan)

Test reliability improvements:
- window.location teardown moved to afterEach (no state leak on failure)
- expect.assertions(N) on all tests (catch silent false passes)
- Shared setWindowLocation helper for consistent location mocking

* ♻️ refactor: Import `buildLoginRedirectUrl` directly from data-provider

Update `AuthContext.tsx` and `useAuthRedirect.ts` to import
`buildLoginRedirectUrl` from `librechat-data-provider` instead of
re-exporting through `~/utils/redirect.ts`.

Convert `redirect.ts` to ESM-style inline exports and remove the
re-export of `buildLoginRedirectUrl`.

* ✅ test: Move `buildLoginRedirectUrl` tests to data-provider

Tests for `buildLoginRedirectUrl` now live alongside the implementation
in `packages/data-provider/specs/api-endpoints.spec.ts`.

Removed the duplicate describe block from the client redirect test file
since it no longer owns that function.

client/src/routes/useAuthRedirect.ts

@@ -1,6 +1,6 @@
 import { useEffect } from 'react';
 import { useLocation, useNavigate } from 'react-router-dom';
-import { buildLoginRedirectUrl } from '~/utils';
+import { buildLoginRedirectUrl } from 'librechat-data-provider';
 import { useAuthContext } from '~/hooks';
 
 export default function useAuthRedirect() {