fix(auth): harden provider request startup

Summary

• Add an Auth helper for looking up the Ueberauth strategy configured for a provider.
• Start the configured provider from AuthController.request/2 when the Ueberauth plug route table falls through.
• Cover the fallback path with a controller regression test using a fake redirect strategy.

Testing

• mix test test/hive_web/controllers/auth_controller_test.exs test/hive_web/live/account_live/identities_test.exs
• mix compile --warnings-as-errors
• mix test
• MIX_ENV=test HIVE_GITHUB_CLIENT_ID=client-id HIVE_GITHUB_CLIENT_SECRET=client-secret mix run -e 'Application.ensure_all_started(:hive); conn = Plug.Test.conn(:get, "/auth/github"); conn = HiveWeb.Endpoint.call(conn, []); IO.inspect({conn.status, Plug.Conn.get_resp_header(conn, "location"), conn.halted})'

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