Function migrate_workspace_env_keys 

pub fn migrate_workspace_env_keys(
    workspace: &Path,
    providers: &[Provider],
    store_mode: AuthCredentialsStoreMode,
) -> Result<(MigrationSummary, Vec<(Provider, MigrationOutcome)>)>

Migrate API keys for multiple providers from workspace .env into secure storage.

This is the batch-optimized path: the .env file is read once, all providers are processed in memory, and the file is rewritten at most once.