[Main] Optimize MPPI visualization to skip processing when no subscribers

[SatArw]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Issue #5801
Primary OS tested on	Ubuntu 24.04.3 LTS (via nav2_dev Docker container)
Robotic platform tested on	Gazebo Simulation (TurtleBot3 Waffle)
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	No

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Description of contribution in a few bullet points

This PR is an adaptation of #5807 for the main (rolling) branch.

• Updated both overloaded instances of TrajectoryVisualizer::add to check subscription counts before executing the marker generation loops.
  • Topics involved: Checks get_subscription_count() on the /candidate_trajectories (MarkerArray) and /optimal_path (Path) publishers.
• When visualize is set to true:
  • 0 Subscribers (Both topics): The method returns immediately. No markers or paths are calculated.
  • Subscriber for /optimal_path only: Generates and publishes the visualization data for only the optimal trajectory.
  • Subscriber for /candidate_trajectories: Generates the visualization data for all the trajectories (including the optimal trajectory).
• Reduces controller_server CPU usage when visualization is enabled but not being actively viewed.
• This is the same optimization previously applied to the Jazzy branch, now ported to main (rolling).

Description of documentation updates required from your changes

• No documentation updates required. This is an internal optimization that does not change the user-facing API or behavior of the visualize parameter.

Description of how this change was tested

• Unit Testing:

  • Ran the nav2_mppi_controller GTest suite locally. All tests pass (100%).
  • Verified that the logic changes did not break existing functionality or style guidelines (passed ament_uncrustify and cpplint).

• Performance Testing:

  • This optimization was previously tested and validated on the Jazzy branch (see related PR [Jazzy] Optimize MPPI visualization to skip processing when no subscribers #5807 for detailed performance metrics).
  • Results demonstrated significant CPU reduction when visualize=true but no subscribers are present.
  • The same logic applies to the main branch with identical expected benefits.
  • Main branch has the additional publish_optimal_trajectory parameter, but this PR's optimization is independent and complementary to that feature.

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists
• Should this be backported to current distributions? If so, tag with backport-*.

[SteveMacenski]

Ill leave for Maurice to review / approve before I merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
nav2_mppi_controller/src/trajectory_visualizer.cpp	100.00% <100.00%> (ø)

... and 134 files with indirect coverage changes

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[SteveMacenski]

Eh actually I think this is good, no reason to bother Maurice, he's a busy guy