fix(dify): add GET method support to /dify/retrieval for health check (#13837)

## Summary
- Add GET method handler to `/api/v1/dify/retrieval` endpoint for Dify
external knowledge base connectivity verification
- GET requests return a simple success response; POST requests retain
existing retrieval logic unchanged

## Problem
When Dify integrates with RAGFlow as an external knowledge base, it
sends periodic GET requests to the retrieval endpoint for
health/connectivity checks. The endpoint only accepted POST, causing
werkzeug to return `405 Method Not Allowed`. After several successful
POST retrievals, the failing GET health checks trigger Dify's circuit
breaker, causing all subsequent requests to fail.

Traceback from the issue:
```
werkzeug.exceptions.MethodNotAllowed: 405 Method Not Allowed: The method is not allowed for the requested URL.
```

## Changes
- `api/apps/sdk/dify_retrieval.py`: Added a separate GET route handler
(`retrieval_health_check`) that returns `get_json_result(data=True)`

## Test plan
- [ ] Verify `GET /api/v1/dify/retrieval` returns `{"code": 0,
"message": "success", "data": true}`
- [ ] Verify `POST /api/v1/dify/retrieval` with valid API key and body
still works as before
- [ ] Verify Dify external knowledge base integration no longer returns
405 errors

Closes #13788

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Asksksn <Asksksn@noreply.gitcode.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

api/apps/sdk/dify_retrieval.py

@@ -29,7 +29,7 @@ from api.db.services.knowledgebase_service import KnowledgebaseService
 from api.db.services.llm_service import LLMBundle
 from api.db.joint_services.tenant_model_service import get_model_config_by_id, get_model_config_by_type_and_name, get_tenant_default_model_by_type
 from common.metadata_utils import meta_filter, convert_conditions
-from api.utils.api_utils import apikey_required, build_error_result, get_request_json
+from api.utils.api_utils import apikey_required, build_error_result, get_request_json, get_json_result
 from rag.app.tag import label_question
 from common.constants import RetCode, LLMType
 from common import settings
@@ -311,3 +311,10 @@ async def retrieval(tenant_id):
             )
         logging.exception(e)
         return build_error_result(message=str(e), code=RetCode.SERVER_ERROR)
+   
+  
+@manager.route('/dify/retrieval', methods=['GET'])  # noqa: F821
+async def retrieval_health_check():
+    """Health check endpoint for Dify external knowledge base connectivity verification."""
+    return get_json_result(data=True)
+