fix(relay): address code review feedback for Phase 2
- [Issue 1]: Remove struct-level #[allow(dead_code)] from AppState and annotate only the two new fields (oauth_signing_keypair and dpop_nonces) individually to avoid suppressing warnings for unrelated fields
- [Issue 2]: Remove unnecessary #[allow(unused_imports)] annotation and the unused import of p256::elliptic_curve::sec1::ToEncodedPoint; the method to_encoded_point is available without explicit trait import
- [Issue 3]: Strengthen test assertions in store_and_retrieve_oauth_signing_key by replacing weak assert\!(\!is_empty()) checks with assert_eq\! using exact expected values for both public_key_jwk and private_key_encrypted