fix: tests

[michele-milesi]

Summary

Describe the purpose of the pull request, including:

• Update the dummy env to return both image and vector observations
• Update tests to check both the CNN and the MLP encoders (and decoders if present)
• Fix algorithms where tests fail

Type of Change

Please select the one relevant option below:

• Bug fix (non-breaking change that solves an issue)
• Update tests

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.