Fix TopicEcho_TEST race condition with Publisher_TEST (backport #761)

[mergify]

🦟 Bug fix

Summary

Both TopicEcho_TEST and Publisher_TEST use the /echo topic. When ctest runs tests in parallel (e.g. ctest -j$(nproc)), messages from one test leak into the other, causing non-deterministic failures.

This PR uses a unique topic name (/test_topic_echo) in TopicEcho_TEST by calling SetTopic() before OnEcho(true). The Publisher plugin's default topic is /echo (which Publisher_TEST validates), so TopicEcho_TEST is the right one to change.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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Generated-by: Claude Code

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This is an automatic backport of pull request #761 done by Mergify.