fix: allow signIn.social by utilizing discovery url

[dvanmali]

Fixes issue of authorization url not being discovered by social provider.

This addition also enables signIn.social beyond just signIn.oauth2.

Closes: #3278

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Summary by cubic

Use OIDC discovery to resolve the authorization URL in generic OAuth, fixing social login flows. Enables signIn.social for providers configured with discoveryUrl.

• Bug Fixes
  • Fetch authorization_endpoint from discoveryUrl when provided.
  • Apply authorizationUrlParams to the discovered authorization URL.
  • Add test covering signIn.social with a discovered auth URL and correct redirect.

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commit: 1ba326f