Fix force_random_password option in Users API

What does this MR do?

Previously the API did not correctly handle the force_random_password option. It was not being treated as one of the 3 viable password- setting options. This adds it as the 3rd, and fixes the test coverage, which was not properly testing this use case.

Background: The API documentation says: "Either password, reset_password, or force_random_password must be specified."

Does this MR meet the acceptance criteria?

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Related to #25802 (closed)