fix(auth_methods): Add checks for duplicate `auth_method` in create API

[ThisIsMani]

Type of Change

• [x] Bugfix
• [ ] New feature
• [ ] Enhancement
• [ ] Refactoring
• [ ] Dependency updates
• [ ] Documentation
• [ ] CI/CD

Description

Currently create will accept and insert auth methods with same type and name, which shouldn't be possible. This PR fixes that.

Additional Changes

• [ ] This PR modifies the API contract
• [ ] This PR modifies the database schema
• [ ] This PR modifies application configuration/environment variables

Motivation and Context

Closes #5160.

How did you test it?

curl --location 'http://localhost:8080/user/auth' \
--header 'Content-Type: application/json' \
--header 'api-key: test_admin' \
--data '{
    "owner_id": "org_qaamgpukifSyBG0AxtYA2x",
    "owner_type": "organization",
    "auth_method": {
        "auth_type": "open_id_connect",
        "private_config": {
            "base_url": "https://dev-28418517.okta.com",
            "client_id": "0oahmmwdmuFvv2pFo5d7",
            "client_secret": "-VIrZZeN_A0SdSpFykAUZ0iMJNpSYQyILcfUmYlmZaLaFK7uRayrEuSvhs-Um5IR"
        },
        "public_config": {
            "name": "okta"
        }
    },
    "allow_signup": false
}'

If this API is hit with same auth_type and name in public_config, then the API will throw the following error.

{
    "error": {
        "type": "invalid_request",
        "message": "User auth method already exists",
        "code": "UR_43"
    }
}

Checklist

• [x] I formatted the code cargo +nightly fmt --all
• [x] I addressed lints thrown by cargo clippy
• [x] I reviewed the submitted code
• [ ] I added unit tests for my changes where possible