Add test for compare method standard error sorting consistency (#2350)

[uzairgheewala]

Description

This PR adds a test to verify that standard errors maintain their proper association with models in the compare method's output, regardless of input order.

The problem of standard errors not properly maintaining their association with models when the input order changed as mentioned in #2350 was resolved in issue #2351 but #2350 was left open for a test to be added to check just that.

Checklist

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2407.org.readthedocs.build/en/2407/

[uzairgheewala]

The ReadTheDocs build appears to be failing due to a documentation system error related to type hints (FilePath not defined). This seems unrelated to the test changes.

Any assistance that could be provided would be greatly appreciated.