Fix: restore embedding model switching for datasets with existing chunks (#14732)

### What problem does this PR solve?

## Problem

During the REST API refactoring (#13690), the
`/api/v2/kb/check_embedding` endpoint was removed and never migrated to
the new RESTful structure. The frontend was pointed to the
`/api/v1/datasets/{id}/embedding` endpoint (which is `run_embedding` — a
completely different function). Additionally, a hard guard was
introduced that rejects any `embd_id` change when `chunk_num > 0`,
making it impossible to switch embedding models on datasets with
existing chunks.

## Root Cause

1. **Missing endpoint**: The old `check_embedding` logic (sample random
chunks, re-embed with the new model, compare cosine similarity) was not
carried over to the new REST API service layer.
2. **Wrong frontend URL**: `checkEmbedding` in `api.ts` pointed to
`/datasets/{id}/embedding` (`run_embedding`) instead of a dedicated
check endpoint.
3. **Overly restrictive guard**: `dataset_api_service.py` line 310
blocked all `embd_id` updates when `chunk_num > 0`. This check did not
exist in the pre-refactor code — it was incorrectly introduced during
the refactor.

## Changes

### Backend

- **`api/apps/services/dataset_api_service.py`**
  - Remove the `chunk_num > 0` hard guard on `embd_id` updates
- Add `check_embedding()` service function: samples random chunks,
re-embeds them with the candidate model, computes cosine similarity,
returns compatibility result (avg ≥ 0.9 = compatible)
  - Add `import re` for the `_clean()` helper

- **`api/apps/restful_apis/dataset_api.py`**
- Add `POST /datasets/<dataset_id>/embedding/check` endpoint following
the new REST API conventions
  - Clean up unused top-level imports (`random`, `re`, `numpy`)

### Frontend

- **`web/src/utils/api.ts`**
- Fix `checkEmbedding` URL from `/datasets/${datasetId}/embedding` →
`/datasets/${datasetId}/embedding/check`

### Tests

-
**`test/testcases/test_http_api/test_dataset_management/test_update_dataset.py`**
- Update `test_embedding_model_with_existing_chunks` to assert success
(`code == 0`) instead of expecting the old `102` error

-
**`test/testcases/test_web_api/test_dataset_management/test_dataset_sdk_routes_unit.py`**
- Update `test_update_route_branch_matrix_unit` to assert
`RetCode.SUCCESS` when updating `embd_id` on a chunked dataset,
replacing the old `chunk_num` error assertion

### Type of change

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

---------

Signed-off-by: noob <yixiao121314@outlook.com>

test/testcases/test_web_api/test_dataset_management/test_dataset_sdk_routes_unit.py

@@ -548,10 +548,11 @@ def test_update_route_branch_matrix_unit(monkeypatch):
 
     kb_chunked = _KB(kb_id="kb-1", name="old", chunk_num=2, embd_id="embd-1")
     monkeypatch.setattr(module.KnowledgebaseService, "get_or_none", lambda **kwargs: kb_chunked if kwargs.get("id") else None)
+    monkeypatch.setattr(module.KnowledgebaseService, "update_by_id", lambda *_args, **_kwargs: True)
     req_state.clear()
     req_state.update({"embd_id": "embd-2"})
     res = _run(inspect.unwrap(module.update)("tenant-1", "kb-1"))
-    assert "chunk_num" in res["message"], res
+    assert res["code"] == module.RetCode.SUCCESS, res
 
     kb_rank = _KB(kb_id="kb-1", name="old", pagerank=0)
     monkeypatch.setattr(module.KnowledgebaseService, "get_or_none", lambda **kwargs: kb_rank if kwargs.get("id") else None)