DefaultCredentialResolver

Non-interactive credential resolution:

• CredentialSource.None → null (no credentials needed)

• CredentialSource.Environment → reads the named environment variables (a missing variable is an ImportException)

• CredentialSource.ExternalFile → reads a TOML file with username and password keys

• CredentialSource.RuntimePrompt → null (no prompt available headless; a front-end resolver overrides this case)