Bug: Service API QA segment updates clear the answer when it is omitted

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### Dify version

1.17.0

### Cloud or Self Hosted

Self Hosted (Source)

### Steps to reproduce

1. Run Dify Self Hosted from source at commit `1299d99b2d70b1b639819248715dd3340979665f`.

2. Create or select a knowledge base with a document in QA mode (`doc_form=qa_model`). Create a completed and enabled segment with content `old question` and answer `old answer`.

3. Using a dataset API key, update only the content through the Service API:

```http
POST /v1/datasets/{dataset_id}/documents/{document_id}/segments/{segment_id}
Authorization: Bearer
Content-Type: application/json

{"segment": {"content": "new question"}}
```

4. Retrieve the segment and inspect `answer`, `status`, `enabled`, and `error`.

5. Repeat with the high-quality indexing technique. A keywords-only partial update also reproduces the answer loss:

```json
{"segment": {"keywords": ["new keyword"]}}
```

This was verified through the real Service API `SegmentService.update_segment` path with a real SQLAlchemy SQLite session.

### ✔️ Expected Behavior

The API should not mutate an existing QA segment when a required `answer` is missing.

Please confirm which contract is intended:

1. If QA updates require an answer, reject the request with a clear HTTP 400 validation error before mutation or indexing, and leave the existing segment unchanged. The schema and API documentation should state that `answer` is required for QA updates.

2. If partial updates are supported, an omitted `answer` should preserve the existing answer. An explicitly supplied answer should replace it, with the behavior for `null` or an empty string documented.

### ❌ Actual Behavior

The request is accepted, but an omitted answer is converted to `None` and written into the segment.

- Economy indexing: the existing answer is persisted as `null` while the segment remains `completed` and enabled. This causes silent data loss.
- High-quality indexing: the update attempts to concatenate the content with `None`, causing `can only concatenate str (not "NoneType") to str`. The exception is caught internally; the segment is persisted as `error` and disabled, while the Service API still returns HTTP 200.
- A keywords-only update has the same answer-clearing behavior.

The relevant code currently allows an optional answer in `SegmentUpdateArgs`:
https://github.com/langgenius/dify/blob/main/api/services/entities/knowledge_entities/knowledge_entities.py#L253

The QA update path then assigns `segment.answer = args.answer`:
https://github.com/langgenius/dify/blob/main/api/services/dataset_service.py#L3713

Local focused verification passed: Service API controller tests (111), SegmentService tests (42), and update_segment focused tests (13). Existing tests did not cover a QA partial update with an omitted answer.

Related context: Issue #6251 documents `answer` as required when creating a QA document; this report concerns the inconsistent behavior of the Service API segment update endpoint.

If maintainers confirm this issue is valid, please assign this issue to me so I can prepare the PR. I will wait for confirmation before opening the PR.

Start with SegmentUpdateArgs in api/services/entities/knowledge_entities/knowledge_entities.py and the QA update path in api/services/dataset_service.py. Run the listed Service API controller, SegmentService, and update_segment tests, then add focused coverage for omitted answers on QA and keywords-only updates. Done means the agreed validation or preservation contract is enforced without mutating the existing segment.

Written by the indexing model from the issue text.