Function is_fallbackable 

pub fn is_fallbackable(err: &LLMError) -> bool

Default fallbackability predicate shared by the fallback layer.

Unlike is_retryable, every LLMError::HttpError is fallbackable regardless of message content so callers can route any transport failure to a backup provider. Retryability still requires a transport-failure message via is_transport_retryable_message.