Hit testing fails with "Can't operate on closed transaction inside context manager"

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### Dify version

1.16.0-rc1

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

1. Deploy Dify `1.16.0-rc1` locally using Docker Compose.
2. Create a knowledge base.
3. Upload a document and wait for indexing to complete.
4. Open the knowledge base retrieval test page.
5. Enter a query that matches one or more indexed segments.
6. Run the retrieval test.

The API returns HTTP 500. The relevant API log is:

```text
sqlalchemy.exc.InvalidRequestError: Can't operate on closed transaction inside context manager.
Please complete the context manager before emitting further commands.

File "/app/api/services/hit_testing_service.py", line 188, in retrieve
return cls.compact_retrieve_response(query, all_documents, session=session)

File "/app/api/services/hit_testing_service.py", line 244, in compact_retrieve_response
"records": cls._dump_retrieval_records(session, records)

File "/app/api/services/hit_testing_service.py", line 72, in _dump_retrieval_records
for document in session.scalars(
select(DatasetDocument).where(DatasetDocument.id.in_(document_ids))
).all()
```

The issue is reproducible when the retrieval returns matching document segments.

### ✔️ Expected Behavior

The retrieval test should complete successfully and display the matched document segments without returning an internal server error.

### ❌ Actual Behavior

The vector retrieval returns matching segments, but the API responds with HTTP 500 when it attempts to load the related document information.

`HitTestingService.retrieve()` calls `session.commit()` and then calls `compact_retrieve_response()`, which executes another query using the same session. Because the controller manages the session inside a transaction context, the explicit commit closes the transaction before the document query is executed.

Changing the internal `session.commit()` to `session.flush()` allows the outer transaction context to manage the final commit and prevents the error.

Start in api/services/hit_testing_service.py, especially retrieve(), compact_retrieve_response(), and _dump_retrieval_records(), then inspect how the controller manages the SQLAlchemy session transaction. Reproduce with the Docker Compose retrieval test using a query that matches indexed segments. Done means the test returns matched document segments without the closed-transaction HTTP 500.

Written by the indexing model from the issue text.