coverage: fix lint (import order + D401) that broke CI

The prior commit's contact-aware escape tripped two ament lints:
- flake8 I100: `from ros_gz_interfaces.msg import Contacts` was placed before
  `import numpy`; moved to its alphabetical slot (after rclpy, before std_msgs).
- pep257/flake8 D401: `_bumper_held_long` docstring wasn't imperative mood;
  reworded "True if ..." -> "Return True if ...".

No behavior change. Verified locally in ros:jazzy: ament_flake8, ament_pep257,
ament_copyright, and tools/check_rosdeps.py all clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: kaiaai

src/oomwoo_coverage/oomwoo_coverage/coverage_planner_node.py

@@ -48,8 +48,6 @@
 
 from nav_msgs.msg import OccupancyGrid
 
-from ros_gz_interfaces.msg import Contacts
-
 import numpy as np
 
 import rclpy
@@ -63,6 +61,8 @@
     QoSReliabilityPolicy,
 )
 
+from ros_gz_interfaces.msg import Contacts
+
 from std_msgs.msg import Bool, Float32
 
 # OccupancyGrid cell conventions
@@ -612,7 +612,7 @@ def _bumper_fresh(self, t) -> bool:
         return t is not None and self._elapsed(t) < self.bumper_fresh_sec
 
     def _bumper_held_long(self) -> bool:
-        """True if a bumper has been pressed *continuously* for the escape threshold."""
+        """Return True if a bumper has been held *continuously* past the threshold."""
         if not self.contact_aware_escape:
             return False
         for last, since in ((self._bump_left_t, self._bump_left_held_since),