Fix: Enforce default embedding model in create_dataset / update_dataset (#8486)

### What problem does this PR solve?

Previous:
- Defaulted to hardcoded model 'BAAI/bge-large-zh-v1.5@BAAI'
- Did not respect user-configured default embedding_model

Now:
- Correctly prioritizes user-configured default embedding_model

Other:
- Make embedding_model optional in CreateDatasetReq with proper None
handling
- Add default embedding model fallback in dataset update when empty
- Enhance validation utils to handle None values and string
normalization
- Update SDK default embedding model to None to match API changes
- Adjust related test cases to reflect new validation rules

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/sdk/dataset.py

@@ -347,6 +347,8 @@ def update(tenant_id, dataset_id):
                 return get_error_data_result(message=f"Dataset name '{req['name']}' already exists")
 
         if "embd_id" in req:
+            if not req["embd_id"]:
+                req["embd_id"] = kb.embd_id
             if kb.chunk_num != 0 and req["embd_id"] != kb.embd_id:
                 return get_error_data_result(message=f"When chunk_num ({kb.chunk_num}) > 0, embedding_model must remain {kb.embd_id}")
             ok, err = verify_embedding_availability(req["embd_id"], tenant_id)

api/utils/validation_utils.py

@@ -380,7 +380,7 @@ class CreateDatasetReq(Base):
     name: Annotated[str, StringConstraints(strip_whitespace=True, min_length=1, max_length=DATASET_NAME_LIMIT), Field(...)]
     avatar: str | None = Field(default=None, max_length=65535)
     description: str | None = Field(default=None, max_length=65535)
-    embedding_model: Annotated[str, StringConstraints(strip_whitespace=True, max_length=255), Field(default="", serialization_alias="embd_id")]
+    embedding_model: str | None = Field(default=None, max_length=255, serialization_alias="embd_id")
     permission: PermissionEnum = Field(default=PermissionEnum.me, min_length=1, max_length=16)
     chunk_method: ChunkMethodEnum = Field(default=ChunkMethodEnum.naive, min_length=1, max_length=32, serialization_alias="parser_id")
     parser_config: ParserConfig | None = Field(default=None)
@@ -435,9 +435,16 @@ class CreateDatasetReq(Base):
         else:
             raise PydanticCustomError("format_invalid", "Missing MIME prefix. Expected format: data:<mime>;base64,<data>")
 
+    @field_validator("embedding_model", mode="before")
+    @classmethod
+    def normalize_embedding_model(cls, v: Any) -> Any:
+        if isinstance(v, str):
+            return v.strip()
+        return v
+
     @field_validator("embedding_model", mode="after")
     @classmethod
-    def validate_embedding_model(cls, v: str) -> str:
+    def validate_embedding_model(cls, v: str | None) -> str | None:
         """
         Validates embedding model identifier format compliance.
 
@@ -464,16 +471,17 @@ class CreateDatasetReq(Base):
             Invalid: "@openai" (empty model_name)
             Invalid: "text-embedding-3-large@" (empty provider)
         """
-        if "@" not in v:
-            raise PydanticCustomError("format_invalid", "Embedding model identifier must follow <model_name>@<provider> format")
+        if isinstance(v, str):
+            if "@" not in v:
+                raise PydanticCustomError("format_invalid", "Embedding model identifier must follow <model_name>@<provider> format")
 
-        components = v.split("@", 1)
-        if len(components) != 2 or not all(components):
-            raise PydanticCustomError("format_invalid", "Both model_name and provider must be non-empty strings")
+            components = v.split("@", 1)
+            if len(components) != 2 or not all(components):
+                raise PydanticCustomError("format_invalid", "Both model_name and provider must be non-empty strings")
 
-        model_name, provider = components
-        if not model_name.strip() or not provider.strip():
-            raise PydanticCustomError("format_invalid", "Model name and provider cannot be whitespace-only strings")
+            model_name, provider = components
+            if not model_name.strip() or not provider.strip():
+                raise PydanticCustomError("format_invalid", "Model name and provider cannot be whitespace-only strings")
         return v
 
     @field_validator("permission", mode="before")