Summary Fixes #260. This PR improves hostname validation in craft_store.auth.Auth to prevent invalid host values from being used when storing or retrieving credentials from the system keyring. Previously, passing a full URL instead of a hostname could result in credential lookup failures because the keyring used a different service identifier. Changes Made * Added hostname validation during Auth initialization. * Rejects invalid host values containing URL schemes, ports, paths, or malformed hostname labels. * Raises a clear ValueError when an invalid hostname is provided. * Updated the integration-test fixture to extract the hostname from the Charmhub base URL. * Added parametrized unit tests covering valid and invalid hostname inputs. Testing * Added unit tests for valid hostname formats. * Added tests for malformed hostnames, URLs, and ports. * The full repository test suite has not yet been run. Expected Behaviour Valid hostnames such as api.charmhub.io are accepted. Invalid inputs such as https://api.charmhub.io and api.charmhub.io:443 are rejected with a descriptive error. This helps prevent credential storage mismatches and makes authentication configuration errors easier to diagnose. Closes #260.