Add a test for recursively deleting nested models

[XINJIANGMO]

🦟 Bug fix

Fixes #782

Summary

Add a test for gz-sim#3080 to avoid regressions of this bug.

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.

[Copilot]

Pull request overview

This PR adds a regression test for a crash bug that occurred when recursively deleting nested models. The bug was originally reported in gz-sim#3080 and fixed in gz-physics#781. The test ensures that removing a parent model with multiple nested models, followed by attempting to remove the already-deleted nested models, doesn't cause a crash due to iterator invalidation.

Changes:

• Added RemoveNestedModelCrash test to verify that removing nested models doesn't crash
• Test creates 6 nested models within a parent model, removes the parent, then attempts to remove the already-removed nested models

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