Add has_usable_secret() to reject empty, short (<4 char), and common
placeholder API key values (changeme, your_api_key, placeholder, etc.)
throughout the auth/runtime resolution chain.
Update list_available_providers() to use provider-specific auth status
via get_auth_status() instead of resolve_runtime_provider(), preventing
cross-provider key fallback from making providers appear available when
they aren't actually configured.
Preserve keyless custom endpoint support by checking via base URL.
Cherry-picked from PR #2121 by aashizpoudel.
f304bc63b8