Introduce a new test case to verify that `UnmarshalString` fails when given an invalid string. This ensures the robustness of the error handling in the `SMTPAuthType` unmarshalling process.
Included the SPDX license identifiers for copyright and license type at the beginning of the auth_test.go file. This ensures clear licensing information is provided and helps with automated license compliance.
Implement the fig.StringUnmarshaler interface for SMTPAuthType to map strings to corresponding authentication types. Added comprehensive unit tests to ensure correct functionality for all supported auth type strings.