Return HTTP 412 status for inactive OIDC auth methods

[davidsbond]

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This commit modifies the OIDC authentication start method to return a 412 (Precondition Failed) error if a client attempts to start the authentication flow against an OIDC auth method that is set to inactive. This is something we noticed in our error logs in HCP Boundary that is impacting our SLOs.

Previously this returned its own error code which is mapped to a 500 response. Tests have been updated to catch this new scenario.