authMethod default not stored in auth options (AO2)

[paddybyers]

Spec point

• AO2: `authMethod` defaults to `'GET'` and should be accessible on the auth options object.

Observed behaviour

When `authMethod` is not explicitly set, `auth.authOptions.authMethod` is `undefined`. The GET default is applied at HTTP request time, not stored in the options object.

Failing test

Test	Expected	Actual
`AO2 - authMethod defaults to GET`	`auth.authOptions.authMethod === 'GET'`	`undefined`

Reproduction

```bash
RUN_DEVIATIONS=1 npx mocha --grep "AO2" test/uts/rest/types/options_types.test.ts
```

From PR #2191, branch `uts-rest`.

┆Issue is synchronized with this Jira Task by Unito