fix(backend): preserve users custom llm settings during settings migrations (#12134)

Co-authored-by: Xingyao Wang <xingyao@all-hands.dev>

openhands/storage/data_models/settings.py

@@ -30,6 +30,7 @@ class Settings(BaseModel):
     llm_model: str | None = None
     llm_api_key: SecretStr | None = None
     llm_base_url: str | None = None
+    user_version: int | None = None
     remote_runtime_resource_factor: int | None = None
     # Planned to be removed from settings
     secrets_store: Secrets = Field(default_factory=Secrets, frozen=True)