test(actions): add unit tests for face action

[LupaFlex]

Summary

Added comprehensive unit tests for the face action module which previously had no test coverage.

Changes

• tests/actions/face/test_interface.py: Tests for FaceAction enum, FaceInput dataclass, and Face interface
• tests/actions/face/connector/test_avatar.py: Tests for FaceAvatarConnector

Test Coverage

• FaceAction enum values and type validation (6 expressions: happy, confused, curious, excited, sad, think)
• FaceInput creation with all action types
• Face interface with input/output handling
• FaceAvatarConnector initialization
• Avatar command mapping for all expressions
• Stop method behavior

Testing

• All 15 tests pass
• Pre-commit checks pass
• Ruff linting passes

Notes

• Follows existing test patterns (no __init__.py files in test directories)
• Uses proper module mocking for external dependencies (zenoh, zenoh_msgs)