Fix unused result warnings (backport #493) (backport #496)

[mergify]

🦟 Bug fix

Fixes compiler warnings, related to gazebosim/gz-transport#810

Summary

As noted in the protobuf 34.0 release notes [[nodiscard]] has been added to many APIs, which is causing several unused-result compiler warnings in this package:

• https://build.osrfoundation.org/job/gz_fuel_tools-ci-gz-fuel-tools11-homebrew-arm64/48/clang/

This checks the result of each affected API to fix the compiler warnings, matching the error response strategy used in nearby code for each instance.

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.
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Generated-by: Remove this if GenAI was not used.

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Backports: If this is a backport, please use Rebase and Merge instead.

---

This is an automatic backport of pull request #493 done by [Mergify](https://mergify.com).

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This is an automatic backport of pull request #496 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c381002 has failed:

On branch mergify/bp/ign-fuel-tools7/pr-496
Your branch is up to date with 'origin/ign-fuel-tools7'.

You are currently cherry-picking commit c381002.
  (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)

Changes to be committed:
	modified:   src/FuelClient.cc
	modified:   src/gz.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/Interface.cc

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

[scpeters]

I'll try again