test(sdkv3): add tests for expiring credentials

[Hweinstock]

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.

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[hayemaxi]

use these

aws-toolkit-vscode/packages/core/src/shared/datetime.ts

Line 6 in 2ff5518

// constants for working with milliseconds