Skip legacy IMDS expiry extension on new refresh path

[alexgromero]

Issue #, if available:
Python-9032

Overview

Updates the IMDS credential-fetching path to use the new shared credential refresh behavior when DEFAULT_NEW_CREDENTIAL_REFRESH is enabled.

On the flag-off path, IMDS keeps the existing legacy expiry-extension behavior. On the flag-on path, IMDS no longer calls _evaluate_expiration(), returns the real IMDS expiry, and relies on the shared RefreshableCredentials refresh-window behavior instead.

Testing

Also updates the flag-on test suite in tests/unit/test_credential_refresh.py to reflect the intended GA migration shape:

• keep mirrored classes as pass when unchanged
• use additive subclass coverage where existing tests still apply
• fully copy the IMDS fetcher test class where legacy expiry-extension tests need to be removed/replaced

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