Backport nonbreaking changes from ionic

[scpeters]

🎉 New feature

Backports non-breaking changes from #2494 and #2500 to harmonic

Summary

This backports the non-breaking changes from #2494 and #2500 to harmonic:

• https://github.com/gazebosim/gz-sim/commit/79ab869af051043bf5c38444d9ee20e1b3b6bee3: add Model::ModelCount and Model::ModelByName nested model accessor APIs
• https://github.com/gazebosim/gz-sim/commit/774f21c15c69c6dec4ef0230df160d8c4db2c121: add WrenchMeasured component. It's not currently used, but it's safe to add.
• https://github.com/gazebosim/gz-sim/commit/501d155ecc1c9584ea20462aac05ffd15d1fd141: since the sensor_TEST.py PreUpdate callback assumes that it executes after the ForceTorque PreUpdate callback, move the test callback to Update to guarantee it. Also fix a spelling error.

The remaining commits make minor non-breaking changes.

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Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.