fix: make control_msgs optional

[dancher00]

Summary

• lazy-load control_msgs only when parallel_gripper_action is selected
• keep the component factory and all non-gripper components importable without control_msgs
• document the optional ROS package and add regression coverage

The source-install commands reported in #2 were already corrected by c8ca08b; this addresses the remaining import-time dependency.

Tests

• pytest tests/test_optional_control_msgs.py -q — 2 passed
• ruff check and ruff format --check — passed
• the pre-existing test_get_state_sees_live_messages passes in isolation but remains order-dependent after test_state_node_is_not_in_the_executor

Fixes #2