fix: padronize models as _models.py

[betinacosta]

Description

In CONTRIBUTING.md we had suggested the models.py naming, but on GUIDELINES.md and our code is suggested and used _models.py. This PR changes the CONTRIBUTING.md and updates DMS naming so both uses _models.py.

Related Issue

#72

Type of Change

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• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

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Checklist

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• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

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• What breaks
• Migration path for users
• Alternative approaches considered

Additional Notes

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