fix(api): move dify retrieval health check to /dify/retrieval/health (#15311)

### Related issues
Closes #15310

### What problem does this PR solve?

`/api/v1/dify/retrieval` had duplicate `GET` route registrations in
`dify_retrieval_api.py`: one for authenticated retrieval and another for
unauthenticated health checks. Sharing the same path and method created
ambiguous routing behavior and an unstable API contract for Dify
external knowledge base integration.

This PR separates concerns by moving the health-check endpoint to `GET
/api/v1/dify/retrieval/health`, while keeping retrieval on
`/api/v1/dify/retrieval`. This makes auth behavior deterministic and
prevents route shadowing/conflicts.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)