Remove underscored objects from rosbag_py tutorial (backport #4976)

[mergify]

Some objects that are available directly from the rosbag2_py root module are imported from the underscored modules they live in. This change removes the underscore modules and uses the objects from the root module.

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This is an automatic backport of pull request #4976 done by Mergify.

[mergify]

Cherry-pick of 9f4a25f has failed:

On branch mergify/bp/jazzy/pr-4976
Your branch is up to date with 'origin/jazzy'.

You are currently cherry-picking commit 9f4a25f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   source/Tutorials/Advanced/Recording-A-Bag-From-Your-Own-Node-Py.rst

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[fujitatomoya]

@christophebedard can you merge this?

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/13042663767/artifacts/2507706456.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4979/index.html in your favorite browser