fix: add hmac feature to libsecp256k1 for test guardian functions

- Add hmac feature to libsecp256k1 in workspace Cargo.toml to enable sign function
- Clean up unused imports (PublicKey, Signature, RecoveryId) in main contract and test module
- Fix compilation error: unresolved import 'libsecp256k1::sign'
- All 27 tests now pass with cargo test --features std
- WASM build maintains compatibility with Stylus runtime
- Test guardian functions (create_valid_guardian_signature) now work correctly

Co-Authored-By: ayush.suresh@dourolabs.xyz <byteSlayer31037@gmail.com>

Author: devin-ai-integration[bot]