[WIP] Update credential support for model service API key management

[Copilot]

Thanks for the feedback on #5. I've created this new PR, which merges into #5, to address your comment. I will work on the changes and keep this PR's description up to date as I make progress.

Original PR: #5
Triggering review: #5 (comment)

@copilot open a new pull request to apply changes based on this feedback

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[OhYee]

In an async test method, Credential.create() should be replaced with await Credential.create_async() to maintain consistency with the async pattern used elsewhere in the test (e.g., await ModelService.create_async(), await ms.delete_async(), await cr.delete_async()). Using synchronous calls in async methods can lead to blocking behavior and defeats the purpose of async testing.