Fix thread issue by adding check for invalid CLI combo

[XINJIANGMO]

🦟 Bug fix

Fixes #3132

Summary

In the previous flow, the GUI thread (which spawns a separate GUI subprocess) starts before createServerConfig runs. So when gz.cc prints the error, the GUI is already launched; returning or calling std::exit in gz.cc won’t stop the GUI subprocess started via std::system.
This order also causes the crash: main exits while the GUI std::thread remains joinable, its destructor calls std::terminate, resulting in terminate called without an active exception.

This fix adds a check for this invalid combo in sim_main.cc to avoid thread issue.

Checklist

• Signed all commits for DCO
• 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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

[arjo129]

Thanks for catching this, it makes sense. LGTM

[iche033]

minor issue picked up by CI:

/github/workspace/src/cmd/sim_main.cc:467:  Lines should be <= 80 characters long