fix: explicitly access enum member values to avoid Python version rel…

[malav-shastri]

Issue #, if available:

Description of changes:

• In python version Python 3.6 and earlier, the default str method for enum members returns the full string representation, including the class name and member name (e.g., 'HubContentType.MODEL'). Which results in an invalid public model arn format and fails to create a hub model reference for those Python versions.
• To maintain consistency across various Python versions, explicitly accessing the value attribute of the enum object to get the value and construct the correct Hub Model Arn.

Testing done:

• Notebook testing

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the Python SDK team
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Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes
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• I have used unique_name_from_base to create resource names in integ tests (if appropriate)

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[sage-maker]

lgtm