Prevent inadvertent submodule downgrades

[speth]

Changes proposed in this pull request

• Restore yaml-cpp submodule to version 0.9.0
• Add a CI job to ensure that submodule downgrades are flagged as an error

If applicable, fill in the issue number this pull request is fixing

Closes #2170

Example usage

I initially opened this PR with a commit downgrading the googletest submodule. The new CI job fails and provides the following log messages:

AI Statement (required)

• Extensive use of generative AI. Significant portions of code or documentation were generated with AI, including
  logic and implementation decisions. All generated code and documentation were reviewed and understood by the contributor. Implemented using Claude Sonnet 5.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• AI Statement is included
• The pull request is ready for review