WaitForTopics: wait for publisher-subscriber connection to be established (backport #474)

[mergify]

Description

In the WaitForTopics class, wait for subscription to be connected to at least one publisher before calling the trigger function.

Is this user-facing behavior change?

No

Did you use Generative AI?

No

Additional Information

This is a continuation of PR #356

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This is an automatic backport of pull request #474 done by Mergify.

[mergify]

Cherry-pick of fb6d0b5 has failed:

On branch mergify/bp/jazzy/pr-474
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	both modified:   launch_testing_ros/launch_testing_ros/wait_for_topics.py

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