Users can now select models with multiple slashes like 'openrouter/google/gemini-2.5-pro'
in the TUI. Previously the TUI would only parse the first two parts of the model
path, showing 'Invalid model openrouter/google' for nested models.
When API errors like token limit exceeded errors are passed as strings to error checking methods, the 'in' operator would throw a TypeError. This fix adds a type guard to check that the input is an object before attempting to access its properties, allowing proper error classification even when encountering unexpected error formats from providers.