Replace fake metrics with description fallback and change loading and error states for validated model cards

[Taj010]

Description

For the Model Catalog:

• The extractValidatedModelMetrics util falls back to hard-coded defaults (fake metrics) is changed to a description fallback
• The spinners is changed to skeletons when loading artifacts inside the body of each cards for validated models on the catalog landing page
• Fix the 500 error state coming from the BFF when loading those artifacts to have it fall back to the non-validated-model version of the card and have a toast notification with "error loading performance artifacts for model"with API error message.

How Has This Been Tested?

Updated validatedModelUtils.spec.ts to verify extractValidatedModelMetrics returns null when performance artifacts array is empty or index is out of bounds

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

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• For first time contributors: Please reach out to the Reviewers to ensure all tests are being run, ensuring the label ok-to-test has been added to the PR.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
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