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 -->

api/utils/validation_utils.py

@@ -362,6 +362,11 @@ class GraphragConfig(Base):
     resolution: Annotated[bool, Field(default=False)]
 
 
+class ParentChildConfig(Base):
+    use_parent_child: Annotated[bool, Field(default=False)]
+    children_delimiter: Annotated[str, Field(default=r"\n", min_length=1)]
+
+
 class AutoMetadataField(Base):
     """Schema for a single auto-metadata field configuration."""
 
@@ -387,6 +392,7 @@ class ParserConfig(Base):
     graphrag: Annotated[GraphragConfig, Field(default_factory=lambda: GraphragConfig(use_graphrag=False))]
     html4excel: Annotated[bool, Field(default=False)]
     layout_recognize: Annotated[str, Field(default="DeepDOC")]
+    parent_child: Annotated[ParentChildConfig, Field(default_factory=lambda: ParentChildConfig(use_parent_child=False))]
     raptor: Annotated[RaptorConfig, Field(default_factory=lambda: RaptorConfig(use_raptor=False))]
     tag_kb_ids: Annotated[list[str], Field(default_factory=list)]
     topn_tags: Annotated[int, Field(default=1, ge=1, le=10)]