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/api_utils.py

@@ -440,6 +440,10 @@ def get_parser_config(chunk_method, parser_config):
                 ],
                 "method": "light",
             },
+            "parent_child": {
+                "use_parent_child": False,
+                "children_delimiter": "\n",
+            },
         },
         "qa": {"raptor": {"use_raptor": False}, "graphrag": {"use_graphrag": False}},
         "tag": None,
@@ -467,16 +471,23 @@ def get_parser_config(chunk_method, parser_config):
     # If no parser_config provided, return default merged with base defaults
     if not parser_config:
         if default_config is None:
-            return deep_merge(base_defaults, {})
-        return deep_merge(base_defaults, default_config)
+            merged_config = deep_merge(base_defaults, {})
+        else:
+            merged_config = deep_merge(base_defaults, default_config)
+    elif default_config is None:
+        # If parser_config is provided but no defaults for this method
+        merged_config = deep_merge(base_defaults, parser_config)
+    else:
+        # Ensure raptor and graph_rag fields have default values if not provided
+        merged_config = deep_merge(base_defaults, default_config)
+        merged_config = deep_merge(merged_config, parser_config)
 
-    # If parser_config is provided, merge with defaults to ensure required fields exist
-    if default_config is None:
-        return deep_merge(base_defaults, parser_config)
-
-    # Ensure raptor and graph_rag fields have default values if not provided
-    merged_config = deep_merge(base_defaults, default_config)
-    merged_config = deep_merge(merged_config, parser_config)
+    # Flatten parent_child config into children_delimiter for the execution layer
+    pc = merged_config.get("parent_child", {})
+    if pc.get("use_parent_child"):
+        merged_config["children_delimiter"] = pc.get("children_delimiter", "\n")
+    elif pc:
+        merged_config["children_delimiter"] = ""
 
     return merged_config