feat(auth): use gnap error middleware on idp api

[njlie]

Changes proposed in this pull request

• Adds the gnapServerErrorMiddleware to the routes on the Identity Provider API on the auth server.

Context

Closes #3029.

This ended up being a one-line change, as the IDP responses were added already in the spec from #850 and #3024. The code already throws these errors and had tests added as of #2400. There was one potential response missing from the spec, however.

Checklist

• [x] Related issues linked using fixes #number
• [ ] Tests added/updated
• [x] Make sure that all checks pass
• [ ] Bruno collection updated (if necessary)
• [ ] Documentation issue created with user-docs label (if necessary)
• [x] OpenAPI specs updated (if necessary)

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