Feat: expose parent-child chunking configuration via HTTP API and Python SDK (#13940)

…
### What problem does this PR solve?

Closes #13857

Parent-child chunking was introduced in v0.23.0 but is only configurable
through the web UI. Users managing datasets programmatically cannot
enable it via the HTTP API or Python SDK because `ParserConfig` uses
`extra="forbid"`, rejecting the `children_delimiter` field at
validation.

### What does this PR change?

Adds a `parent_child` nested config to `ParserConfig`, following the
same pattern as `raptor` and `graphrag`:

```json
"parser_config": {
  "parent_child": {
    "use_parent_child": true,
    "children_delimiter": "\n"
  }
}
```

- api/utils/validation_utils.py — new ParentChildConfig model, added to
ParserConfig
- api/utils/api_utils.py — naive defaults + flatten to
children_delimiter for the execution layer
- api/apps/services/dataset_api_service.py — flatten on the update path
- test/testcases/configs.py — updated DEFAULT_PARSER_CONFIG
-
test/testcases/test_http_api/test_dataset_management/test_create_dataset.py
— 4 valid + 2 invalid test cases

No changes to the execution layer (rag/app/naive.py, rag/nlp/search.py).
Existing UI flow via ext is unaffected.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added parent-child chunking configuration for dataset creation and
updates with new `use_parent_child` toggle and customizable
`children_delimiter` setting to specify how parent chunks are split into
child chunks.

* **Documentation**
* Updated HTTP and Python API references with parent-child chunking
configuration details and examples.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

docs/references/python_api_reference.md

@@ -187,7 +187,7 @@ The chunking method of the dataset to create. Available options:
 The parser configuration of the dataset. A `ParserConfig` object's attributes vary based on the selected `chunk_method`:
 
 - `chunk_method`=`"naive"`:  
-  `{"chunk_token_num":512,"delimiter":"\\n","html4excel":False,"layout_recognize":True,"raptor":{"use_raptor":False}}`.
+  `{"chunk_token_num":512,"delimiter":"\\n","html4excel":False,"layout_recognize":True,"raptor":{"use_raptor":False},"parent_child":{"use_parent_child":False,"children_delimiter":"\\n"}}`.
 - `chunk_method`=`"qa"`:  
   `{"raptor": {"use_raptor": False}}`
 - `chunk_method`=`"manuel"`:  
@@ -480,7 +480,7 @@ A dictionary representing the attributes to update, with the following keys:
   - `"email"`: Email
 - `"parser_config"`: `dict[str, Any]` The parsing configuration for the document. Its attributes vary based on the selected `"chunk_method"`:
   - `"chunk_method"`=`"naive"`:  
-    `{"chunk_token_num":128,"delimiter":"\\n","html4excel":False,"layout_recognize":True,"raptor":{"use_raptor":False}}`.
+    `{"chunk_token_num":128,"delimiter":"\\n","html4excel":False,"layout_recognize":True,"raptor":{"use_raptor":False},"parent_child":{"use_parent_child":False,"children_delimiter":"\\n"}}`.
   - `chunk_method`=`"qa"`:  
     `{"raptor": {"use_raptor": False}}`
   - `chunk_method`=`"manuel"`: