deps(sdkv3): add tests for refreshing credentials on client.

[Hweinstock]

built on changes in https://github.com/aws/aws-toolkit-vscode/pull/5940/files

Problem

The client should not be making requests with stale credentials so we want test coverage to ensure this.

Solution

• Create a mock credentials shim and insert it into the global context.
• The client builder will then use that shim, and we can manually expire it.
• Ensure client does not cache potentially stale credentials.

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License: I confirm that my contribution is made under the terms of the Apache 2.0 license.