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/http_api_reference.md

@@ -566,6 +566,9 @@ curl --request POST \
       - Defaults to: `{"use_raptor": false}`
     - `"graphrag"`: `object` GRAPHRAG-specific settings.
       - Defaults to: `{"use_graphrag": false}`
+    - `"parent_child"`: `object` Parent-child chunking settings. When enabled, each chunk is further split into smaller child chunks using `children_delimiter`. At retrieval time, matched child chunks are replaced by their parent's full text before being passed to the LLM, giving precise vector matching with broader context.
+      - `"use_parent_child"`: `bool` Whether to enable parent-child chunking. Defaults to `false`.
+      - `"children_delimiter"`: `string` The delimiter used to split a parent chunk into child chunks. Only takes effect when `"use_parent_child"` is `true`. Defaults to `"\n"`.
   - If `"chunk_method"` is `"qa"`, `"manuel"`, `"paper"`, `"book"`, `"laws"`, or `"presentation"`, the `"parser_config"` object contains the following attribute:  
     - `"raptor"`: `object` RAPTOR-specific settings.
       - Defaults to: `{"use_raptor": false}`.
@@ -820,6 +823,9 @@ curl --request PUT \
       - Defaults to: `{"use_raptor": false}`
     - `"graphrag"`: `object` GRAPHRAG-specific settings.
       - Defaults to: `{"use_graphrag": false}`
+    - `"parent_child"`: `object` Parent-child chunking settings. When enabled, each chunk is further split into smaller child chunks using `children_delimiter`. At retrieval time, matched child chunks are replaced by their parent's full text before being passed to the LLM, giving precise vector matching with broader context.
+      - `"use_parent_child"`: `bool` Whether to enable parent-child chunking. Defaults to `false`.
+      - `"children_delimiter"`: `string` The delimiter used to split a parent chunk into child chunks. Only takes effect when `"use_parent_child"` is `true`. Defaults to `"\n"`.
   - If `"chunk_method"` is `"qa"`, `"manuel"`, `"paper"`, `"book"`, `"laws"`, or `"presentation"`, the `"parser_config"` object contains the following attribute:  
     - `"raptor"`: `object` RAPTOR-specific settings.
       - Defaults to: `{"use_raptor": false}`.