AppModelConfig has so many nullable fields

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### Dify version

master

### Cloud or Self Hosted

Self Hosted (Source)

### Steps to reproduce

https://github.com/langgenius/dify/pull/30919

### ✔️ Expected Behavior

not so many nullable fields

### ❌ Actual Behavior

```
id: Mapped[str] = mapped_column(StringUUID, default=lambda: str(uuid4()), init=False)
app_id: Mapped[str] = mapped_column(StringUUID, nullable=False)
provider: Mapped[str | None] = mapped_column(String(255), nullable=True, default=None)
model_id: Mapped[str | None] = mapped_column(String(255), nullable=True, default=None)
configs: Mapped[Any | None] = mapped_column(sa.JSON, nullable=True, default=None)
created_by: Mapped[str | None] = mapped_column(StringUUID, nullable=True, default=None)
created_at: Mapped[datetime] = mapped_column(
sa.DateTime, nullable=False, server_default=func.current_timestamp(), init=False
)
updated_by: Mapped[str | None] = mapped_column(StringUUID, nullable=True, default=None)

opening_statement: Mapped[str | None] = mapped_column(LongText, default=None)
suggested_questions: Mapped[str | None] = mapped_column(LongText, default=None)
suggested_questions_after_answer: Mapped[str | None] = mapped_column(LongText, default=None)
speech_to_text: Mapped[str | None] = mapped_column(LongText, default=None)
text_to_speech: Mapped[str | None] = mapped_column(LongText, default=None)
more_like_this: Mapped[str | None] = mapped_column(LongText, default=None)
model: Mapped[str | None] = mapped_column(LongText, default=None)
user_input_form: Mapped[str | None] = mapped_column(LongText, default=None)
dataset_query_variable: Mapped[str | None] = mapped_column(String(255), default=None)
pre_prompt: Mapped[str | None] = mapped_column(LongText, default=None)
agent_mode: Mapped[str | None] = mapped_column(LongText, default=None)
sensitive_word_avoidance: Mapped[str | None] = mapped_column(LongText, default=None)
retriever_resource: Mapped[str | None] = mapped_column(LongText, default=None)
prompt_type: Mapped[str] = mapped_column(
String(255), nullable=False, server_default=sa.text("'simple'"), default="simple"
)
chat_prompt_config: Mapped[str | None] = mapped_column(LongText, default=None)
completion_prompt_config: Mapped[str | None] = mapped_column(LongText, default=None)
dataset_configs: Mapped[str | None] = mapped_column(LongText, default=None)
external_data_tools: Mapped[str | None] = mapped_column(LongText, default=None)
file_upload: Mapped[str | None] = mapped_column(LongText, default=None)
```

I think this need to be splited.