Function resolve_model 

pub fn resolve_model(
    model_str: &str,
    default_provider: CliRunnerType,
) -> ResolvedProvider

Parse a model string into a provider type and optional model name

Formats supported:

• "copilot:gpt-4o" → (Copilot, Some(“gpt-4o”))
• "claude:opus" → (ClaudeCode, Some(“opus”))
• "copilot" → (Copilot, None) — use provider default model
• "gpt-4o" → (default_provider, Some(“gpt-4o”)) — no prefix, use server default