Function resolve_whisper_model_path_with_override 

pub fn resolve_whisper_model_path_with_override(
    config: &AppConfig,
    session_model: Option<&str>,
) -> Result<PathBuf, AppError>

Resolve the path to a local Whisper model file, with an optional session-scoped override.

This is the core-owned implementation of the GUI’s per-session model selection rules.

When session_model is provided (and non-empty after trimming), it is interpreted as:

• a full path when it contains a path separator (/ or \\), otherwise
• a filename resolved under AppConfig::whisper_models_dir().

If a session override is provided but the resolved file does not exist, this returns an error instead of silently falling back to global defaults.

When no session override is provided, this falls back to resolve_whisper_model_path.