[ulleo]

Summary

Fixes #1409. Starting Q&A from a datasource in a type-4 page-embedded assistant (busiFlag: 'ds') now opens Chat inside the embedded page and supports returning to the datasource list.

 Start Q&A from an embedded datasource
- /chat/index → /login → /chat/index → …
+ /embeddedCommon → embedded Chat → return to datasource list

The loop occurred because protected routes required a normal login token, while the login redirect accepted the UID loaded by embedded authentication. Require both the token and UID for that redirect. Preserve the selected datasource ID, embedded handshake query parameters, and datasource list state; retain existing route allowlists and permission checks.

Evidence

• Before: the four new regression cases fail on base e028a237; real Vue Router alternates between Chat and login until the test's navigation limit stops it.
  After: on ad90677a, python -m pytest -q backend/tests/test_embedded_datasource_navigation.py backend/tests/test_embedded_auth_bypass_fix.py passes 44 tests. The tests execute source handlers, actual Vue Router, and the compiled embedded component with isolated service/child-component stubs. They cover normal login, denied routes, model-check failure, embedded context, datasource ID precision, and returning/reopening Chat.
• frontend: npm run build and ESLint for the four changed Vue/TypeScript files pass. Ruff check/format pass for the new regression file. Existing local dependencies were reused without syncing (Vue 3.5.40, Vue Router 4.6.4, TypeScript 5.7.3).
• Manual browser verification reported by the tester: after adding the local embedded user, clicking “开始问数” worked without the freeze. Browser verification did not cover returning to the datasource list or the full SQL/LLM pipeline; those are not claimed as end-to-end passes. No image deployment was performed.

Merge Danger

Door: two-way

Frontend-only behavior change; no migration, dependency, or embedded authentication protocol change.

Blast Radius: navigation

Affects the embedded datasource-to-Chat transition and the normal-login redirect when a UID exists without a login token.