Create subscriber on constructor

[Tacha-S]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#4802
Primary OS tested on	Ubuntu
Robotic platform tested on	gazebo simulation of Tally
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

• I modified it to create a subscriber during initialization while maintaining the functionality to switch topics based on the input port status.

Description of documentation updates required from your changes

• There are no changes to the inputs, outputs, parameters, or overall behavior, so there is no need to update the documentation.

Description of how this change was tested

• Create a tree with IsBatteryLow as the top-level condition and invoke the tree when the battery level is low.

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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

[mergify]

@Tacha-S, your PR has failed to build. Please check CI outputs and resolve issues.
You may need to rebase or pull in main due to API changes (or your contribution genuinely fails).

[SteveMacenski]

I approve of these changes, but I think this should be made for all BT nodes so that this behavior is consistent across the board. Plus, I'm sure you'll use other BT nodes and want this done for them anyway, so may as well save yourself the headache and batch it out now :-)

[Tacha-S]

Understood. I will check and fix the other BT nodes as well.