saveApiKey function
Save an API key to secure storage and config.
Implementation
Future<void> saveApiKey(String apiKey) async {
if (!isValidApiKey(apiKey)) {
throw ArgumentError(
'Invalid API key format. API key must contain only '
'alphanumeric characters, dashes, and underscores.',
);
}
// Placeholder: full implementation stores in platform keychain or config.
}