[Safety] Zero velocity should always be published on controller failure (backport #5279)

[mergify]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#5273
Primary OS tested on	(Ubuntu 24.04)
Robotic platform tested on	(TB4 Simulation)
Does this PR contain AI generated software?	(Yes)

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Description of contribution in a few bullet points

• Modified onGoalExit() in nav2_controller to accept a bool force_stop argument.
• Ensured that a zero velocity is always published in cases of failure or exception, regardless of the publish_zero_velocity parameter setting.
• Improves safety behavior by guaranteeing that the robot stops on controller failure, cancellation, or internal exception.
• Updated all exit points in computeControl() to call onGoalExit(true) on failure and onGoalExit(false) on success.

Description of documentation updates required from your changes

• No new parameters introduced.
• Minor clarification could be added to the publish_zero_velocity parameter documentation:

  This parameter only affects zero-velocity publishing on successful goal completion. Failures or cancellations will now always publish zero velocity regardless of this setting.

Description of testing

Tested using the Nav2 TurtleBot4 simulation:

ros2 launch nav2_bringup tb4_simulation_launch.py headless:=False

• Sent a navigation goal via RViz, then canceled it mid-execution.
• Verified that /cmd_vel_nav published a zero velocity immediately after cancellation:

ros2 topic echo /cmd_vel_nav 

• Confirmed that a zero velocity was sent even when publish_zero_velocity := false.

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Future work that may be required in bullet points

• Consider introducing an explicit always_stop_on_failure parameter for full user control over emergency stop policy.

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

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This is an automatic backport of pull request #5279 done by [Mergify](https://mergify.com).

[mergify]

@mergify[bot], all pull requests must be targeted towards the main development branch.
Once merged into main, it is possible to backport to @kilted, but it must be in main
to have these changes reflected into new distributions.