test: cover nebula_hesai_common helpers

[mojomex]

Summary

• add focused unit tests for nebula_hesai_common
• cover PTP conversions, Hesai return-mode mappings, support predicates, and stream operators
• wire the package into ament_cmake_gtest so the tests run in CI

Verification

• source /opt/ros/humble/setup.bash && colcon build --packages-select nebula_hesai_common --symlink-install --mixin debug compile-commands coverage-gcc
• source /opt/ros/humble/setup.bash && colcon test --packages-select nebula_hesai_common --event-handlers console_direct+

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.57%. Comparing base (ac59892) to head (fc87d34).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   39.72%   47.57%   +7.84%     
==========================================
  Files         128      163      +35     
  Lines       10018    13302    +3284     
  Branches     4977     6964    +1987     
==========================================
+ Hits         3980     6328    +2348     
- Misses       5443     5588     +145     
- Partials      595     1386     +791     

Flag	Coverage Δ
nebula_hesai_common	74.00% <ø> (?)

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