Add Failure API tests for user defined federated authenticator management

[Shenali-SJ]

Proposed changes in this pull request

This PR adds API tests to validate failure scenarios for IdPs with user-defined authenticators.

Related Issue:

• Provide API for the new custom authentication management #21216

Related PR:

• [Draft] Add integration tests for managing user defined federated authenticator #21646

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General

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Functionality

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Tests

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Documentation

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• Are reusable components documented? If this PR introduces components that are relevant to other developers (for instance a mixin for a view or a generic form) they should be documented in the Wiki.

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